Nitrate is certainly less toxic to fish than either ammonia or nitrite and until recently considered to be harmless unless in very high concentrations.
Studies have since shown that although nitrate may not be lethal unless in exceptionally high quantities it does make fish more susceptible to disease and reduces the likelihood of breeding success.
The other problem with high nitrates is the possibility of inducing nitrate shock to new fish when introducing them to a tank containing higher nitrates than they are accustomed to. As with nitrite, some fish are more sensitive to the effects of nitrate than others.
A level of 0 mg/l (zero parts per million) in a freshwater tank is difficult to achieve however one must try to maintain as low a nitrate level as possible. Certainly levels above 20 mg/l should be avoided for sensitive fish such as Discus and levels below 40 mg/l should acceptable for most other fish.
Check nitrates regularly with a test kit.
With the advances made in the aquarium hobby in the last few decades, aquarium filtration types have reached what would have been considered impossible levels of effectiveness.
Today one can purchase filtration systems which can perform mechanical, chemical, and biological filtration in one unit, as well as the accompanying oxygenation.
One can even purchase a nitrate filter which can effectively eliminate the end product of biological filtration. These filters require additional maintenance and are easily neglected.
There is an alternative nitrate filter which is not unsightly, requires minimal maintenance and tolerates a fair degree of neglect – live plants.
Live plants are an excellent filtration system, as well as being attractive in a tank. Most fast growing plants are relatively heavy feeders and so are quite effective at removing nitrate from the aquarium water.
Some good examples of nitrate removing plants are:
- Hygrophila species and Amazon Swordplants for freshwater
- Caulerpa species for marine aquaria.
However, keeping plants in the tank with your fish is not always feasible.
An example would be if the tank’s occupants were largely herbivorous, like pacus or tangs. In this case a reasonable alternative may be to create a type of “algae tank.”
This could be as simple as having a well lit tank close enough to the tank to be filtered for a circulation of water between the two tanks.
This effectively separates the plants from the plant-eaters, while allowing the plants ready access to the nitrate-rich water.
There are many ways to accomplish the circulation. The simplest being a side-to-side arrangement with equal water levels, simple siphon arrangement, and a powerhead to return the water from tank to tank.
This filtration system has some additional benefits, including additional gallonage per fish and ready supply of natural food for your herbivorous fish and inverts, giving them a healthier diet.