There is less written about the possibly deadly consequences of nitrates in the aquarium than about ammonia and nitrites in the water.
Nitrate is less poisonous to fish than ammonia and nitrites, but it can still kill your fish. This is known as Nitrate Poisoning and also Nitrate Shock.
Waste produced by fish builds up in your tank if it is not cared for properly. Dirty filters if not cleaned correctly from decaying food and plant material combined with over-feeding and overstocking the tank add to increased nitrates.
A build-up of nitrates can lead to fish being more vulnerable to disease, it reduces their capacity to reproduce and can stunt their growth.
Nitrate poisoning occurs gradually with rising nitrate levels. Regular tank cleaning and proper maintenance will prevent this happening.
Not all fish react the same to nitrates, some will be affected by levels as low as 20 mg/l, while others will show no symptoms until levels have reached 200 mg/l.
Before adding water, test it for nitrates so you know if the levels are unusually high in your water source. If nitrates are above 10 ppm, you should consider other water sources that are free of nitrates.
The lower the better. Nitrate levels are generally low naturally in fresh water, subject to it not being contaminated with plant and animal debris, generally well below 5 mg/l. (5 ppm)
In freshwater aquariums it is recommended to keep nitrates below 25 mg/l and certainly never over 50 mg/l.
Saltwater aquarium are different, some experts say that you can have levels slightly higher up to 40 mg/l for fish but corals and other invertebrates will suffer. It is best to consider 25 mg/l as the top limit. Please be aware that if you have a reef tank with invertebrates, you will want to keep the nitrate below 5 mg/l for their good health.
If you are breeding fish, or are suffering with algae growth, you must keep nitrates below 10 mg/l. (10 ppm).
Nitrate shock happens when fish are exposed to a much higher levels of nitrate, or when nitrate levels suddenly drop. Some aquarists say that a sudden or rapid reduction of nitrates is just as harmful to the fish as high nitrate levels. But not everyone finds this to be true, however, I need to warn you that this may be a problem. Just like nitrate poisoning, immature young fish and certain species, Discus being notable, are more sensitive to sudden changes in nitrate levels.
Increased levels of nitrates promote algae growth. This can cause undesirable algae blooms in new tanks. Although plants use nitrates, if the nitrate level rises faster than the plants can use them, the plants can become overgrown with algae, causing their death.
When you realise your tank nitrate level is too high, you need to establish the actual cause.
Questions to ask yourself;
If you have not been maintaining your tank correctly, then a few steps can easily get you back in control. Starting by changing the above issues.
However, if you have been looking after your tank with good maintenance and tank husbandry then the level of nitrates in the tank can never realistically be lower than that of the water you put in it to start.
If the water from your tap was high in nitrates then you must treat it before using it and take further steps for the water already in your tank.