Cardinal Tetras Dieing

[Lady Hobbs]

21 small fish in a 30 gallon tank should not be spiking nitrates to 40 either and you should not have to be doing water changes 3 times a week. Sounds like an overfeeding problem to me.

Nitrates can also raise if the filter is too dirty and needs a rinsing. You didn't mention the filter so I would also wonder if enough oxygen is getting to the fish.

[VRB]

Lady Hobbs. Do you think that 21 fish in a 30 gallon tank is a little on the high side for this capacity fish tank. I know this depends on tank filtration and maintenance as well. Cardinals are fairly sensitive overall. It could be more then one issue leading to death. It could be multiple issues like: stress, not enough oxygen, nitrates, high mineral content, inadequate filtration. Like I said cardinals are a little sensitive

[Lady Hobbs]

No because cardinals are very small and are low-bioload. Harleys are also small fish.