New Tank Syndrome
When fish start to die in an aquarium set up for less than a month, it is often called “New Tank Syndrome.” Toxic levels of either ammonia or nitrite cause new tank syndrome in an aquarium without a mature nitrogen cycle. Typically, if fish show stress or start to die within two weeks or less, it is a toxic level of ammonia. At two to four weeks after the aquarium is set up with fish, nitrite poisoning typically causes stress and mortality.