Nitrate (NO3) is a nitrogen by-product of the nitrifying bacteria (Nitrospira) in a filter or substrate consuming nitrite. It was previously thought species of Nitrobacter did this. but since 1998 it's considered to be species of Nitrospira.
- Nitrate is far less toxic than nitrite and much less toxic than ammonia.
The level of Nitrate in water that may damages aquatic animals varies considerably in each species and at their stage of growth. With mortality of eggs and very young fry being sensitive to low levels (~20mg/l) of nitrate.  Also nitrate toxicity is made further complex as fish can become accustomed to a slowly growing level of nitrate over time and remain seemingly visually unharmed.
Scientific studies into nitrate toxicity have been primary been performed on commercial fish consumed by the public. For example fish like Salmon, trout and some large catfish. With the majority of the fish used in the ornamental pet trade remaining untested and therefore an unknown factor.
The safe level of nitrate varies considerably between species and its age. The hobby has settled for an average maximum level of 50mg/l in a typical community tropical tank. But this level may need to be adjusted downwards if you wish to own known nitrate sensitive fish like Stingrays or Discus. Marine fish owners often set a safe level of below 20mg/l.
Symptoms of Nitrate poison
It inhibits growth, damages internal organs and impairs the immune system in young fish. In older fish high levels cause stress leading to a depressed immune system, behavioural changes and even blindness and death.
- There is no single level 'dangerous' dosage. Aquarists playing safe by trying to keep this level below 50mg/l. Though in reality species vary enormously in their tolerance to this toxin. The theory today is that prolonged exposure to elevated levels of nitrate may decrease the immune response, induce internal hematological and biochemical changes within the animal (behaviour changes) and may increase mortality. Especially in fry
- The average fish can withstand quite high dosages of nitrates (100-500mg/l) as long as the build up of the chemical is slow and over many days in the tank. However it's quite common for less experience aquarists to go and introduce a new fish to their seemingly healthy tank of fish only for the new fish to die over night due to nitrate poisoning if they're tank water is heavy with nitrate. And as stated previously each species tolerance is different (and poorly researched in the cases of ornamental aquatic pets) which is why the hobby often recommends an average safe level of 50mg/l.
Testing for Nitrates
There are many test kits available from pet shops for testing the level of nitrate in water. Some are simple strips of cards which you dip into water. Other more accurate ones use drops of chemicals you mix with a sample of water.
- Aquarists should always have a test kit on hand. If there is an unexplained death or behaviour in the tank then an immediate test of nitrates, nitrites, etc. will be required as soon as possible in case action needs to be carried out.
If levels are not going down then you may need to do several things:
- Do regular weekly water tests so you are not caught unaware.
- Do a water change. By simply performing a 25% water change once a week, most tanks will remain under the recommended 50mg/l level.
- Check the nitrate level in your tap water. Some freshwater suppliers may supply tap water to you with a high level of nitrate already!
- Add a few fast growing plants to the tank. Aquatic plants consume nitrates as it's one of their nutrients. Though they may also need other nutrients in order to remove nitrates.
- Reduce feeding, you may be overfeeding the animals.
- Do not overstock your tank. Remove some animals if need be.
- Add a chemical treatment that removes or neutralises the nitrates.
- Add a nitrate reactor which removes the nitrate by chemical or bacteria process.
- Add bacteria (mainly bacillus sp. bacteria) that removes nitrate by consuming it. 
In a well planted tank with a good CO2 supply, the plants will want to consume more nitrate than there is in the water. In these situations it is often the case that aquatics manually add nitrate via chemical powders.
- Typical nitrate chemical is Potassium nitrate or less commonly Sodium nitrate.
- Tolerance of developing salmonid eggs and fry to nitrate exposure
- Acute and chronic toxicity of nitrate to early life stages of lake trout (Salvelinus namaycush) and lake whitefish (Coregonus clupeaformis)
- Nitrate Toxicity: A Potential Problem of Recirculating Systems (Archived web link Nov. 2004)
- Enterobacter sakazakii, Bacillus coagulans, Bacillus subtillis, Bacillus sphaericus, Bacillus megatarium, Bacillus licheniformis, Bacillus cereus, Bacillus pasteurii, Bacillus cirroflagellosus, Bacillus pumilus. As listed in US Patent 6025152.
- Patent US6025152 - Denitrifying bacterial preparation and method