Sources

Nitrates are:

• produced as Nitrobacter bacteria breaks down nitrites;
• introduced in tap water (in some areas of the UK, tapwater nitrate levels exceed 130mg/l);
• plant materials or run off from cultivated ground

Nitrate is the end product of the break-down of ammonia. Thus its presence indicates that the filter is functioning. In many cases the absence of nitrate is more worrying, as it may indicate that the filter is not functioning.

Measurement of nitrate

Test kits are available (Some measure Nitrate-Nitrogen, a conversion factor then being applied to obtain a true Nitrate reading; full instructions should be available with the kits used).

Safe levels of nitrate

Nitrate is generally of low toxicity though some species, especially marines, are sensitive to its presence. When nitrate levels are high, as a result of biological filtration, other toxici chemicals produced in this process may be present at levels that adversely affect fish health.

In the "Water Quality Criteria", OATA recommends that nitrate levels in freshwater systems do not exceed those in the tap water supply by 50mg/l. As the livestock are more sensitive in marine systems, nitrate should not exceed that in the water supply by 40mg/l.

Reduction of high levels of nitrate

High levels of nitrate can be reduced by:

• dilution by water change
• use of ion exchange materials
• increasing plant density
• use of denitrifying biological filtration