From The Aquarium Wiki
What is it
- Sometimes called 'new fish syndrome'.
When you have an aquarium with fish, they can become used to the toxic chemical nitrate which is produced by your filter bacteria. If you are slack about performing water changes or you have a heavily stocked tank this level can rise quickly.
Nitrate is far less toxic than ammonia or nitrite. But it can still kill if high enough. As levels climb the existing fish become used to it at first, levels can rise to 100ppm or even 150ppm before the fish start to become ill and die as their immune systems become depressed and they get infections they can't fight off.
- Fish species vary enormously on the levels of nitrates they can withstand. It is standard practise in the hobby to try to keep nitrate levels below ~50ppm with 20ppm or lower being ideal levels.
- Owning a nitrate test kit is one essential item a fish owner should never be without.
Often the inexperienced aquarist first discovers this problem when they add a new fish. The fish often becomes seriously ill and dies within hours of being added. It has had no time to slowly become accustomed to the nitrate.
So before you introduce new fish do a nitrate test (hopefully with a test kit under 6 months old). If it's over 50 ppm do a water change to get it under 20ppm.