And now for an answer to your question.
Cut and pasted from deep within my own favorite library
How toxic is nitrate?
There are several different opinions on what represents a safe nitrate level for different varieties of fish and invertebrates. But, in truth, nobody really knows what levels are dangerous for the majority of species. Nitrate toxicity is is still a poorly-researched area, especially with regards to the long-term effects.
What we do know, though, is that few fish are adapted to live in water containing nitrate. Most natural habitats, with the exception of those polluted by man, are relatively nitrate-free. Therefore, you should try to maintain your nitrate level as low as you possibly can.
Importantly, research has shown that it’s more toxic in saltier water, so brackish species and marine fish in poorly maintained aquaria are more at risk than tropical freshwater fish in dirty tanks. But that’s not an excuse to neglect your community tank...
The Ornamental Aquatic Trade Association (OATA) have a set of water quality criteria for aquatic shops. They recommend keeping nitrate levels within 50 mg/l of the levels found in the tapwater supply for freshwater species, and within 40 mg/l of the levels in tapwater for marines. It is recommend keeping the level as low as you can, and below 10 mg/l if you keep sensitive fishes, or inverts.
And some more blubs:
How does nitrate affect my fish?
Nitrate is the least toxic of the elements of the nitrogen cycle and the effects on fish are normally due to long term exposure.
Some of the effects can be summarized as follows:
Poor growth
Loss of appetite
Lethargy
Chronic stress
Reluctance to breed
Increase in gill rate and gasping
General ill health
Delayed wound healing
Clamped fins
And this one:
Nitrate removal
> Dilution
Dilution is the main method used to control nitrate. The trick is ensuring you change enough water regularly enough to control the continual rise in nitrate. Having source water with a low nitrate level helps - use RO water
> Chemical media
Ion-exchange media take in nitrate ions and swap them for others like sodium. These media can be added to external power filters in freshwater only, and can be recharged in strong salty water.
> Enzymes
Enzymes (biological catalysts which speed up reactions) like STMs A-Z Nitrate, are claimed to aid the breakdown of nitrate in both marine and freshwater.
> Natural filtration
Natural filtration techniques based on live rock, plenum, deep sand beds and mud refugiums harbour bacteria capable of reducing nitrate. They are among the best forms of control in the reef tank.
> Biological media
Some special biological media are designed to encourage the growth of bacteria. These help to convert the ammonia and nitrites quickly into nitrates, however you still need a method to deal with the nitrates. Some even try to promote the growth of anerobic bacteria, but with limited sucess.
> Vegetable filtration - BOBS PERSONAL FAVORITE
Algae and aquatic plants feed on a range of nitrogen sources including nitrate, which is why algae proliferates in polluted tanks. Nitrate accumulates in the vacuoles of marine algae and in plant tissues, so by encouraging vegetation to grow and periodically removing it from the system you can help remove nitrate.
> Denitrators
Denitrators are special filters designed to promote the growth of anaerobic bacteria in a very low oxygen environment. These can be very effective, but they need careful setting-up and monitoring otherwise they could start churning out nitrite.
Hope that help answer your question.
Thomas