Effect of extremely hard water on marine life?

Effect of extremely hard water on marine life?

We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

When researching water quality standards, I found that a minimum and maximum was set for water hardness. Several sources also state that water can in fact be too hard. The question is how does hard water harm marine life? I understand that it is required to reduce the toxicity of Hydrogen ions and other trace metals, but cannot find anything concerning the effect of water that is too hard.

Hard water helps in direct and indirect ways.

Indirectly, hard water plays an important role in regulation of pH. Respiration and other physiological activities release substances into water that change the water pH and the ions that cause hardness act as buffers and help maintain pH.

Directly, they determine the ease with which osmoregulation occurs. The harder the water, the lesser the influx of water. So the fishes act like an open system and are influenced by the surrounding water.

There are very few fish who absolutely need soft water to survive. Since your pH and hardness are pretty low, it allows you to raise multiple fish from South America. You could do tetras, dwarf cichlids and New World cichlids, Plecos as well.

Potentially discus and altum angelfish as well if you're up to the huge challenge.

However, livebearers will not tolerate this type of water. That goes the same for African cichlids.

Otherwise hard water is completely fine for marine life and other aquatic species.



Hope this information helped!


If you look at the Wiki text on Hard Water, Marine hard water is not mentioned once. That's because it's a term used mostly to qualify freshwater.

Sea water doesn't vary alot in the oceans, only in inland seas and near estuaries and glacial melt, it's called brackish water, brine, not soft seawater.

Sea water which undiluted is called salty, highly saline, high salinity, water, not Hard Seawater.

Marine life isn't affected by the stone type mineral content of the seawater as much as by the highly reactive salts, both sodium and chloride are very harmful to life compared to most stones a the less soluble mineral salts.

Seawater contains a lot more calcium generally than most river water, even though some river water can be completely saturated in calcium and have very high PH. Some fish can adapt to a wide range of soft water hardness, and some fish can adapt to very salty water, but most fish are specialists who don't live in wildly varying ecosystems, only in the sea and in constant softness freshwater.

Read some aquarium chemistry experiments. they are aquariumabulous and will give you a lot of knowledge about marine biogeochemistry. It's pretty interesting to look at any pond and river and plant and know what chemistry it has, what it needs, and the processes changing it, it's quite simple to learn too, just learn some basic experiments and guides on water and pedology organic and mineral reactions and biology.


Water Hardness Measurement

Water hardness is the total calcium and magnesium ion concentration in a water sample and is expressed as the concentration of calcium carbonate. Temporary hardness is that part of the total hardness that disappears on boiling. Whilst not being accepted as a standard method, the use of ion-selective electrodes allows a rapid measurement of water hardness and can be used to determine changes in hardness. The direct potentiometric method is not recommended for the ion-selective electrode but an indirect potentiometric method involving ethylenediaminetetraacetic acid titration is recommended. The ion-selective electrode that is used is a liquid ion-exchange electrode that responds to the divalent ions magnesium and calcium.

Hard and Soft Water

Water may be polluted in different ways. It effects enormous on aquatic life, environment and human beings. Some effects of water pollution are described below:

Effects of oxygen demanding wastes
The bacterial degradation of waste in water requires oxygen. In this process the bacteria consume the dissolve oxygen in water. If there are huge amount of oxygen demanding wastes, then the oxygen concentration of water may drop so low that it is difficult to survive for the aquatic lives. On the other hand some anaerobic microorganisms begin to bloom. Since they generate harmful toxins as like ammonia and sulfides so it may harmful to people and animals.

Effects of plant nutrients
Plant nutrients as like nitrates, phosphates, potassium may be added to the water bodies through human activities and over nutrition may cause extreme growth of algae and other water plants. In this process a good number of aquatic lives decay moreover as the plant and algae die, it may collect a good amount of organic matters and is filled with sediment. It lowers the oxygen level in the water. Furthermore, to biodegrade this sediment again lowers the oxygen level such a phenomenon is known as eutrophication. It disturbs the ecological balance.

Effects of organic pollutants
Organic pollutants as like oil, plastics, petrochemicals and pesticides are harmful to humans and all plants and animals in the water. Plastic waste can soak up toxic chemicals from ocean pollution. If any marine birds or animals consume, it may reduce appetite or even starvation. Oil is lighter than water it does not mix with water, as an alternative it form a thick layer on the surface. It works as a barrier for photosynthesis hence it is harmful for aquatic life, fish and birds.

Effects of inorganic pollutants
Water soluble inorganic pollutants as like acids, salts and toxic metals make water unfit to drink and will cause the death of aquatic life. Toxic metals as like lead, mercury can cause health and environmental problems, including humans. Many non-metallic substance as like Sulphur is harmful for aquatic life.

Effects of radioactive compounds
Water soluble radioactive compounds are extremely dangerous for health. They can cause cancer, birth defects and genetic damage.

Effects of thermal pollution
Thermal pollution of the water bodies can decreases oxygen levels and may have a terrible effect on aquatic life.

Effects of microorganisms
If Disease-causing microorganisms (pathogenic microbes) are found in surface waters, it may cause human health problems including typhoid, diarrhea, hepatitis and may have negative impacts on aquatic life. Sewage often carries harmful viruses and bacteria into the water bodies and environment.

Effects of agricultural waste
Many agricultural farms often use large amounts of toxic substances as like herbicides and pesticides. These chemicals are particularly dangerous to aquatic life in water bodies.
Asbestos fibers are another serious pollutant it can cause asbestosis, mesothelioma and cancer.
Effects of water pollution are the major problem for developing countries. They do not supply enough pure water. Moreover they are not concern about effects of water pollution.

Effects of water hardness on the physiological responses to chronic waterborne silver exposure in early life stages of rainbow trout (Oncorhynchus mykiss)

Early life stages of rainbow trout were exposed to 0, 0.1 and 1 microg/L Ag (as AgNO(3)) in very soft water (2mg/L CaCO(3)), moderately hard water (150 mg/L CaCO(3)) and hard water (400mg/L CaCO(3)) of low dissolved organic carbon concentration (0.5mg C/L) from fertilization to swim-up (64 days) under flow-through conditions, and monitored for whole embryo/larval silver accumulation, Na(+) and Cl(-) concentrations, Na(+) uptake and Na(+)K(+)-ATPase activity. The objective of the study was to investigate potential protective effects of water hardness on the physiological responses to chronic silver exposure. In the absence of silver, there was little effect of hardness on the ionoregulatory parameters studied, though higher hardness did improve survival post-hatch. At all three water hardness levels, whole embryo/larval Na(+) uptake was low and relatively constant prior to 50% hatch, but dramatically increased following 50% hatch, whereas Na(+)K(+)-ATPase activity steadily increased over development. Whole embryo/larval Na(+) and Cl(-) concentrations were low and constant prior to 50% hatch, but following 50% hatch Na(+) concentration increased, while Cl(-) concentration decreased. Following 50% hatch, exposure to 0.1 and 1 microg/L Ag resulted in a decrease in whole embryo/larval Na(+) concentration, Cl(-) concentration, Na(+) uptake and Na(+)K(+)-ATPase activity, indicating that the mechanism of chronic silver toxicity involves an ionoregulatory disturbance, and is similar to the mechanism of acute silver toxicity. An increase in water hardness reduced or eliminated the effect of silver on these parameters while enhancing survival, suggesting that the nature of the protective effect of hardness involves effects on the ionoregulatory disturbance associated with silver exposure. An increase in water hardness did not fully protect against the accumulation of silver associated with silver exposure. These results suggest that it may be possible to model chronic silver toxicity using a biotic ligand type model, and that a physiologically based model may be more appropriate because Na(+)K(+)-ATPase activity or Na(+) uptake is an endpoint for prediction rather than whole embryo or larval silver accumulation.

What is Hard Water?

Hard water reduces the effect of soap and detergent and causes limescale build-up, but provides a useful source of calcium and magnesium. What softeners are available?

When water is referred to as “hard” or “soft”, it is the number of dissolved minerals that are being described. Tap water in different geographical areas seems to have different properties, especially relating to the effect of detergents, and to the quantity of limescale produced in household appliances.

Dissolved Minerals

The water in rivers, lakes, or under the ground, as well as the water that comes out of the tap, is not pure water, there are small amounts of other chemicals dissolved in it. Some of these chemicals come from the rocks and soils that the water has passed through since it has fallen from the sky like rain. Rainwater is slightly acidic because it has some carbon dioxide dissolved in it, so when it passes through rocks containing calcium carbonate, calcium sulfate, or dolomite (calcium magnesium sulfate), some of these compounds are dissolved into the water.

Caused by Limestone and Chalk

This means that water from areas where there is a lot of limestone (containing mostly calcium carbonate, but also dolomite) or chalk (even higher quantities of calcium carbonate), has a lot of calcium, magnesium, carbonate, hydrogen carbonate, and sulfate ions present in the water – making it hard water. Areas, where there are no limestone or chalk rocks, have much fewer dissolved minerals, and so the water in these areas is called soft water.

Disadvantages of Water Hardness

It is easiest to identify water hardness by its effect on soap and other detergents. Because soaps and detergents have an ionic nature, when they are dissolved in hard water, each soap molecule reacts with calcium ions, to produce scum. This essentially renders the soap useless, and much more soap or detergent is required to clean anything if used with hard water.

Another major problem with hard water is that, when it is heated, in a kettle, washing machine, or hot water pipe, it begins to deposit solid calcium carbonate. This is the cause of limescale building up in kettles, damaging heating elements of washing machines, and blocking water pipes.

Advantages of Water Hardness

Hard water does have some advantages, however. It is an important source of calcium for teeth and bones, as well as other minerals necessary for healthy living. It also does not dissolve so many undesirable substances, so its use to make beer and other beverages produces better results.

Water Softeners

To reduce the negative effects of hard water, many ways have been developed to remove hardness. These techniques range from adding softening chemicals to laundry in order to avoid having to use large quantities of detergent. Portable filters can be used to filter water used in kettles, to prevent scale build-up on the heating element.

Water softeners work using a technique called ion exchange, whereby calcium and magnesium ions are replaced by sodium ions, which do not cause hardness. More recently magnetic water conditioners have been developed which use a magnetic field to alter the way the calcium and magnesium ions behave, reducing the formation of scale.

Frequently Asked Questions

What are the most trusted water heater for hard water brands?

Bosch and Stiebel Eltron are two of the most trustworthy water heaters for hard water brands you can buy. Established in 1886, Bosch is a global name in engineering and technology solutions. You can expect its water heaters to be top-notch.

Stiebel Eltron is another German company with almost 100 years of experience making innovative heating products. Other noteworthy water heater brands include Rinnai, EcoSmart, Titan, Chronomite, and GASLAND.

If you want to use other water heater brands, reading consumer reports can give you a clue as to a brand’s real-world performance.

Do I really need a special water system for hard water?

The need for a special water system for hard water depends on how ‘hard’ the liquid running through your plumbing system is. For example, the US Geological Survey considers a fluid with no more than 60 mg/L of calcium carbonate as ‘soft’. If the CaCO3 is between 61 and 120 mg/L, you can look at it as ‘moderately hard’.

Any CaCO3 reading beyond 180 mg/L is ‘very hard’. The higher the CaCO3 level present in your water, the greater is the risk of limescale formation. This chalky plaque can wreak havoc in your water heaters, forcing the heat exchanger to work hard and using more energy than necessary, increasing your electric or gas bill.

That is why it is always a good idea to have a water softener or any other technology that can prevent CaCO3 from forming limescale in your plumbing system and water appliances, such as a tanked or tankless water heater.

Can you use tankless water heater with hard water?

Ideally? No, you should not use hard water with a tankless water heater. Fluids that contain at least 180 mg/L of CaCO3 can form limescale in the pipes and the tankless device’s heat exchanger. Limescale can reduce the tankless hot water heater element’s heating efficiency, requiring higher energy to heat your water than usual. It can increase your electric or gas bill.

If you have to use a gas or electric tankless hot water heater with hard water, our best advice is to look for a hot water heater that has a superior-quality heat exchanger and heating elements. The more durable these components are, the more resistant they are against limescale formation.

Another solution is to place a water softener in your plumbing system before the water goes through the hot water heater. There are also water conditioners that you can install, such as an electromagnetic unit or a catalytic scale inhibitor system.

How to care & clean?

The amount of cleaning and caring for your hot water heater depends on the device type. Tankless water heaters are pretty straightforward to care for because there are no storage tanks to worry you. However, it is best to keep the tankless water heater free of dust and dirt. Regular checks by a licensed electrician are also a must.

If you have a water heater gas system, checking the burners and gas pipes is crucial for ensuring safety and optimum operation.

A hot water heater with a tank requires periodic flushing and cleaning as frequently as once every three to four months. Removing sediments and other debris from the tank is essential for maintaining its optimum working condition.

Where to buy?

The majority of consumers today choose Amazon when buying almost anything, including a hot water heater for hard water. Amazon not only has the most extensive collection of heaters. It also has exceptional customer service, warranty, and delivery.

Other noteworthy platforms to buy a tanked or tankless water heater include Sears, Lowes, and Home Depot. You may also want to check your local home appliance store that carries domestic heating solutions. Affiliates can also help you.

The effects of water hardness upon the uptake, accumulation and excretion of zinc in the brown trout, Salmo trutta L.

Trent Polytechnic, Department of Life Sciences, Clifton Lane, Nottingham NG118NS, U.K.

Trent Polytechnic, Department of Life Sciences, Clifton Lane, Nottingham NG118NS, U.K.

Trent Polytechnic, Department of Life Sciences, Clifton Lane, Nottingham NG118NS, U.K.

Severn Trent Water Authority, District Fisheries Office, Shelton, Shrewsbury SY38BJ, U.K.

Trent Polytechnic, Department of Life Sciences, Clifton Lane, Nottingham NG118NS, U.K.

Trent Polytechnic, Department of Life Sciences, Clifton Lane, Nottingham NG118NS, U.K.

Trent Polytechnic, Department of Life Sciences, Clifton Lane, Nottingham NG118NS, U.K.

Severn Trent Water Authority, District Fisheries Office, Shelton, Shrewsbury SY38BJ, U.K.


The effects of water hardness (9 and 220 mgl −1 as CaCO3) upon zinc exchange in brown trout exposed to 0.77 μmol Zn 1 −1 have been investigated using artificial soft water (<49.9 μmol Ca l-1 , <40.1 μmol Mg 1 −1 ) and mains hard water (1671.7 μmol Ca 1 −1 , 493.6 μmol Mg 1 −1 ) of known composition. Both hard and soft water-adapted fish exhibited a bimodal pattern of net zinc influx. Net zinc influxes during both fast and slow uptake phases were significantly greater (P<0.001) in soft (82.9 and 6.2 μmol Zn 100 g −1 h −1 ) than in hard water (46.3 and 2.4 μmol Zn 100 g h −1 ). Zinc efflux (- 0.2 μmol Zn 100 g −1 h −1 ) was enhanced only in hard water during the slow net influx phase.

Brown trout exposed to zinc in hard water and placed in metal-free media exhibited a greater net efflux (- 25.6 μmol Zn 100 g −1 h −1 ) of the metal than did fish in soft water (-4.2 μmol Zn 100 g −1 h −1 ) treated in the same manner. Tissue 65 Zn activities reflected both the differences in uptake and excretion rates of the metal between hard and soft water fish. During zinc exposure (0.77 μmol Zn 1 −1 ) high water hardness reduced tissue burdens of the metal by reducing net branchial influx, and enhancing efflux of the metal in hard water fish.

How to Soften Water

Hard water can be softened (have its minerals removed) by treating it with lime or by passing it over an ion exchange resin. The ion exchange resins are complex sodium salts. Water flows over the resin surface, dissolving the sodium. The calcium, magnesium and other cations precipitate onto the resin surface. Sodium goes into the water, but the other cations stay with the resin. Very hard water will end up tasting saltier than water that had fewer dissolved minerals.

Most of the ions have been removed in soft water, but sodium and various anions (negatively charged ions) still remain. Water can be deionized by using a resin that replaces cations with hydrogen and anions with hydroxide. With this type of resin, the cations stick to the resin and the hydrogen and hydroxide that are released combine to form pure water.

The Cost of Hard Water

Hard water causes scale to accumulate in pipe systems and equipment. This water hardness, scale build-up comes at a significant cost.

  • Scale buildup compromises energy-efficiency. For example, in water heating and cooling systems, an eighth of an inch of scale buildup results in a 20 percent loss of efficiency. This means scale causes energy costs to increase significantly.
  • Scale buildup in pipe systems damages the pipes and cause leaks. The subsequent costs of fixing or replacing the pipe system can be very significant. Because of these leaks, there can be severe water damage to the building, including mold growth.
  • Scale buildup shortens the life of residential appliances and commercial/industrial equipment because it reduces water flow and encourages corrosion. Many equipment operators utilize acid cleaning regimens to remove scale accumulation. In conjunction with dissolving the scale, this method gradually destroys metal components and reduces the equipment&rsquos life-cycle.
  • Water hardness can cause sensors, such as flow meter, not to operate adequately.
  • Scale buildup causes significant downtime to manufacturing operations.

Effect of extremely hard water on marine life? - Biology

Nitrite and the Reef Aquarium

M ost aquarists have some familiarity with nitrite. It is part of the "nitrogen cycle" that takes place in most aquariums, and so is one of the first encounters that many aquarists have with their aquariums' chemistry. The marine aquarium hobby is replete with commentary about nitrite, some of which is, unfortunately, incorrect or misleading. Its toxicity in marine systems is far lower than in freshwater systems. Nevertheless, many aquarists incorrectly extrapolate this toxicity to reef aquariums and suggest that any measurable amount of nitrite is a concern.

In reality, nitrite probably is not toxic enough to warrant measuring in most marine systems. This article serves to provide a backdrop for that opinion by addressing what nitrite is, where it comes from, where it goes, the mechanisms by which it can be toxic and the evidence for its toxicity (or lack thereof) in typical reef aquariums.

N itrite (NO 2 - ) is a fairly small ion, consisting of a central nitrogen atom with two attached oxygen atoms in a bent configuration (Figure 1). One of the oxygen atoms carries a negative charge. More correctly, the nitrite ion has two oxygen atoms that are capable of carrying the negative charge, and in reality, the solvated ion in solution probably has two identical oxygen atoms, each with a partial negative charge. Nitrite is a fairly strong acid, and becomes protonated to nitrous acid (HNO 2 ) only as the pH drops below 4 (pKa = 3.35).

In the ocean, nitrite typically varies in concentration from very low levels to about 0.2 ppm. 1,2 The higher end of this range is typically found only in anoxic layers deep below the surface. Nitrite in surface Atlantic and Caribbean seawater has been reported to range from 0.000005 to 0.00002 ppm, 3 and a series of measurements in the South China Sea and the Philippine Sea showed an average of 0.00002 ppm. 4

Nitrite is sometimes elevated in the water buried in sediments due to decomposing organic material, and the fact that such pore water is often anoxic. In natural coral reef sediments, however, nitrite can still be very low (much lower than ammonia (NH 3 ) and nitrate (NO 3 - ), which can rise to as high as 0.7 ppm). 5

Where Does Nitrite Come From?

M ost aquarists associate nitrite with the traditional "nitrogen cycle." In this process, bacteria convert ammonia into nitrite and then into nitrate by oxidizing it. The bacteria gain chemical energy in this fashion, just as other organisms (from bacteria to people) gain energy by oxidizing carbon compounds (such as ethanol, CH 3 CH 2 OH) into more oxidized versions, such as carbon dioxide (CO 2 ).

This process can be described as starting with ammonia (NH 3 ) excreted by animals or by bacteria and other organisms that are consuming organic compounds containing nitrogen, such as proteins. The ammonia from the water column is taken up by bacteria and is oxidized in a step-wise fashion, first to nitrite:

And then to nitrate (possibly in bacteria species other than those that produce the nitrite):

During an aquarium's initial setup, few of these ammonia- and nitrite-oxidizing bacteria are present. As the ammonia accumulates, bacteria that utilize it increase in population. As that occurs, they consume the initial ammonia spike, and a nitrite spike results. Then, the nitrite-oxidizers take advantage of the nitrite spike, increase in population, and consume the nitrite, thereby producing nitrate.

After some period of time (often a few weeks), the bacterial action begins to equilibrate, and neither ammonia nor nitrite is present in high concentrations. This doesn't mean that a lot of each is no longer being produced, only that they are consumed as fast as they are produced, leaving a low steady-state concentration. In most reef aquaria, the steady-state concentrations of both ammonia and nitrite are quite low (less than 0.1 ppm), and often are below the detection limits of many test kits.

What happens when an aquarium is initially set up, however, is not necessarily what happens later. Many organisms in reef aquaria consume ammonia and nitrite directly, and metabolize it into organic matter. Macroalgae, for example, can take up ammonia directly, and many species actually take up ammonia preferentially to nitrate. Consequently, in a reef aquarium such as mine where most of the nitrogen export is via macroalgae, little nitrite may be produced in the first place. I have no way of knowing how much of the nitrogen added to my aquarium from foods enters the macroalgae as ammonia, and how much in other forms (such as nitrite or nitrate), but it is very likely that not all of the nitrogen added passes through a nitrite stage before becoming part of the macroalgae.

In addition to the standard nitrogen cycle, there are other ways that nitrite can be produced. One of these ways is by photolysis of nitrate. That is, nitrate can break apart when exposed to UV light, producing nitrite and hydroxyl radical (OH). 6,7

Another method of nitrite synthesis can occur inside organisms, although this nitrite may not be released back into the water. For example, nitrite can be produced from nitrate internally by corals (e.g., Pocillopora damicornis) and macroalgae (e.g., Ulva lactuca). 8

N itrite can take a number of different pathways in the ocean. Many organisms can directly take up nitrite. Such uptake has been demonstrated in anemones (Condylactis sp., for example, take up nitrite, possibly for its symbionts), 9 diatoms (Eucampia zodiacus) 10 and zooxanthellae isolated from a variety of species (Zoanthus spp., Tridacna crocea, Seriatopora hystrix, Montastrea annularis, Porites furcata and Stylophora pistillata). 11

Nitrite can also be broken down by exposure to UV light, producing nitric oxide (NO), hydroxyl radical (OH) and hydroxide ion (OH - ). 6,7,12

In a laboratory situation with nitrate-free seawater with no organisms present, ambient sunlight can reduce the nitrite concentration by 2-15% per day. 12,13 The primary products of this reaction are nitric oxide (NO) and hydroxyl radical (OH). Both of these compounds are chemically and biologically active, so this reaction may be important to a number of biochemical pathways in the ocean and in various organisms. The effect of nitric oxide is discussed in more detail later in this article.

In the anammox process, bacteria use nitrite to oxidize ammonia, producing N 2 :

The importance of this process in marine sediments has long been unknown. In recent studies, however, it has been shown to be important in some circumstances. 14-17 In two continental shelf sites, the conversion of ammonia to N 2 by this pathway produced 24% and 67% of the total N 2 produced. In a eutrophic bay, however, this process was negligible compared to ordinary denitrification (the conversion of nitrate into N 2 when the nitrate is used as an electron acceptor for degradation of organic material in low oxygen situations). A different study showed that this process accounted for between 4% and 79% of the N 2 produced in coastal sediments.

Finally, in a reef aquarium, nitrite can be removed by reaction with ozone, presumably to produce nitrate. 18

A s described above, nitrite can break down under UV light to produce nitric oxide. Consistent with this process, nitric oxide is found to increase during the day and to decrease at night. 12 Nitric oxide itself has a variety of different biological effects. Exposure to different concentrations of supplemental nitric oxide was found to speed or inhibit the growth of four species of phytoplankton (Skeletonema costatum, Dicrateria zhanjiangensis nov. sp., Platymonas subcordiformis and Emiliania huxleyi) , consistent with its known role as a growth regulator in terrestrial plants. 19

Nitric oxide also may play a role in the symbiosis of certain cnidarians with dinoflagellates. An enzyme that produces nitric oxide has been found in the cnidarian Aiptasia pallida. This enzyme is apparently downregulated when the organism goes into acute heat shock, and inhibitors of the enzyme cause retraction of the tentacles, as is observed under heat shock conditions. 20 Further, addition of nitric oxide donors to the system prevents this retraction of tentacles. Whether this process has anything to do with nitrite or nitric oxide in the water column is not clear.

The effects, if any, that nitric oxide and this reaction from nitrite in particular might have on reef aquaria is unclear. Nitric oxide effects on marine organisms is an active area of research, and a greater understanding of it is expected in the future. Whatever the effects are, however, any effect attributable to NO produced from nitrite may be most pronounced in a newly cycling reef aquarium (where nitrite is elevated) and when a UV system is in use.

N itrite can be toxic in a number of ways. 21 Freshwater fish rapidly take up nitrite through their gills, leading to high levels in their bodies. In freshwater fish, nitrite taken up through the gills can compete with chloride for the same uptake proteins, so in some cases of elevated nitrite the fish can suffer from chloride depletion. It has been observed that some freshwater fish (e.g., bluegill Centrarchidae: Lepomis macrochirus) do not take up chloride via their gills, and these species are notably resistant to nitrite toxicity. 22

The internalized nitrite then causes a number of internal disturbances, including loss of potassium from certain tissues (such as skeletal muscle) and the oxidation of hemoglobin into methemoglobin, which reduces the blood's oxygen carrying capacity. This can cause reduced tissue oxygenation, hyperventilation and heart rate increases. Many other biochemical pathways become altered as well, including steroid synthesis, vasodilation (blood vessel enlargement) and changes in internal levels of ammonia and urea. Nitrite detoxification in freshwater fish is accomplished by direct nitrite excretion and by internal conversion of nitrite into nitrate. 23

Marine species are less susceptible to nitrite toxicity because chloride (at 19,350 ppm in seawater) outcompetes nitrite for the same uptake mechanisms. Nevertheless, it is possible for some marine fish to take up nitrite via both their gills and their intestines after swallowing seawater. For example, when exposed to 46 ppm nitrite in seawater, the European flounder (Platichthys flesus) takes up 66% of its nitrite via intestinal routes. 24 Further, its internal nitrite concentration was found to remain below the ambient nitrite level in the water. At these concentrations, there was some alteration of internal biochemical parameters (such as an increase in methemoglobin levels from 4% in nonexposed fish to 18% of hemoglobin in exposed fish). Nevertheless, there were no mortalities under these conditions, and the difference between this result and what is often observed in freshwater fish at similar nitrite concentrations is attributed to differences in their internal nitrite concentrations.

How Toxic is Nitrite to Fish?

F or the reason described above, nitrite is considerably more toxic to many freshwater fish (Table 1) than it is to most marine species (Table 2). The data in these tables are primarily the LC 50 , which is the concentration at which 50% of the test organisms die (24-h LC 50 is the concentration that kills half of the tested organisms within 24 hours). As Table 1 shows, some freshwater fish can die at nitrite levels below 1 ppm. This toxicity is the reason many aquarists worry about nitrite in aquaria. It can be a significant problem in freshwater aquaria. Tests in marine species, however, showed the toxicity to be much lower. None of the thirteen marine fish species for which I could find nitrite toxicity data had LC 50 values below 100 ppm, and half had LC 50 values of 1,000 - 3,000 ppm or more.

Death is, of course, a very crude indicator of toxicity. An aquarium's nitrite level should not come anywhere close to the LC 50 value, because less severe toxicity can occur even at levels below that. In the previous section, I showed data on one marine species in which biochemical effects could be detected at levels well below concentrations that caused death. We saw, for example, a rise in methemoglobin at values as low as 46 ppm nitrite. However, the point remains valid that marine species are orders of magnitude less susceptible to the effects of nitrite than are many freshwater species. The marine aquaculture industry often uses a rough guideline that the safe rearing level of many compounds is a factor of 10 or less than their LC 50 . 30

In examining ammonia, nitrite and nitrate toxicity in marine species, one might think to look at the effects on larval fish to see if they are more sensitive. In examining the incidence of the larvae's first feeding after hatching, and the 24-h LC 50 , it was found that for seven different marine species, only ammonia was found to be toxic at concentrations that might possibly be encountered in aquaculture facilities. 25

Table 3 brings out the distinction between freshwater and seawater organisms most clearly. In these tests, two fish and one shrimp species that are able to live in both freshwater (or brackish water) and seawater were tested for toxicity at different salinities. At least for these three species, it is clearly shown that nitrite is much more toxic in freshwater (or at lower salinity) than in seawater, even to the same species.

In the only published article 26 that I could find showing toxicity tests to typical reef aquarium fish, Tom Frakes and Bob Studt exposed tank-raised clownfish (Amphiprion ocellaris Figure 2) to nitrite concentrations ranging from 0 to 330 ppm in artificial seawater. Two of five fish died after a few days at 330 ppm, giving an LC 50 not appreciably different from the other species listed in Table 1. At 33 ppm (the next dose down from 330 ppm), the fish were lethargic and breathing with difficulty, but otherwise experienced no lasting problems. At 3.3 ppm nitrite no effects were observed.

One of the difficulties with interpreting toxicity issues, as related by hobbyists who claim to have seen nitrite toxicity in marine fish, is the possible presence of ammonia. In any aquarium with elevated nitrite, the ammonia level also may be elevated. Since ammonia is known to be very toxic to marine fish (LC 50 value below 1 ppm), on the aquarist must ensure that the observations are not flawed by such contaminants. In all of the toxicity tests described above, nitrite is added directly to the seawater, and ammonia would not be expected to be present at significant concentrations, whereas in aquariums the levels of the two materials are not independent of one another.