This nitrate condition is extremely toxic to fish that live in water with high pH values.

The breakdown of waste products by bacteria is commonly called the nitrate cycle.

The bacteria develop on all available surfaces in the aquarium. This includes the glass sides of the aquarium, the rocks and decorations, the substrate and especially in the filter. The only requirements for development are oxygen via the water and a source of nutrition. When the bacteria are fully developed, they convert fish waste to nitrate with no nitrate at all occurring in the aquarium.

IT'S INEVITABLE
Every closed circuit aquarium (one where the water is continually recirculated and/or filtered) will develop, waste assimilating bacteria. When the bacteria are established and of course, provided the bacteria are properly cared for (explained later), then the aquarium will run, trouble free, for an indefinite number of years.

Some aquariums with substrate filters have run continually without completely being dismantled for periods of 15 years and more.

LEVELS OF TOXICITY
Levels as small as 1ppm (Part Per Million) nitrate are toxic to fish. Invertebrates do not seem to be so affected by nitrate as a general rule but while nitrate is present, waste is not being properly treated, so water quality will be down and a drop in water quality could effect invertebrates.

The levels of nitrate produced during biological development are in relation to the (A) quantity of waste available, (B) the extent of surface available to develop on, (C) and the quantity of water in the system. Thus a system with a small surface area but a large volume of water may only produce 5ppm nitrate, where as a small aquarium with a full substrate filter and several centimetres of fine substrate may produce 25ppm nitrate or greater in relation to a similar level of waste.

MINIMISING NITRATE TOXICITY
If a nitrate condition occurs in a stocked aquarium, the toxicity of the nitrate can be minimised by pH control.For marine fish, lower the pH to 7.5. Marine fish can tolerate a low pH for many weeks, where as a pH above 7.5, coupled with a nitrate level of 1ppm or more, will result in death.

ESTABLISHING BIOLOGICAL FILTRATION
Selection of the filtration medium and type of filtration, also methods of biological induction are the key points.

FILTRATION TYPES
The prime requirements for successful filtration are (1) design that permits a continuous flow of oxygenated water through a filtration medium. (2) Design that minimises areas where water flow is absent. Substrate filters and Trickle (Mini-reef) filters are the most common and acceptable types at this time.

FILTRATION MEDIUMS
The considerations for filtration mediums are (1) a surface area large enough to support an adequate quantity of waste assimilating bacteria. (2) A medium that won't change in nature and resists blocking be detritus and silt.

ESTABLISHMENT
Irrespective of the type of filtration or circulation, bacteria is best developed before live stock is cultured. The key attributes of the bacteria are they adhere and develop. Many types of bacteria can assimilate waste in recirculated water. Some do it in the water column. The type required in this case actually adhere to the surfaces in the aquarium system. They also have to develop the ability to convert waste to a less harmful nitrate. During a stage of this development, they convert waste to toxic nitrate.

REOCCURRING NITRATE
A nitrate condition usually reoccures in a newly established aquarium, even after a first and prolonged nitrate condition. This is called a nitrate bounce. The addition of further decor, substrate, living rock or even the cleaning of the decor may cause a bounce as bacteria develop on the new surfaces. Usually nitrate bounces are short lived, but if the fish show signs of distress lower the pH to lessen the toxicity.