goat68gto wrote:The problem remains what caused it to insure that it does not happen again.
Sorry you lost your fish. It's always distressing and disappointing when that happens. Other than pH, what other water parameters do you test for?
Have any of the other fish shown any symptoms at all? If the Rummynoses are happy the water must be OK, those things are quite sensitive (I have a happy healthy shoal downstairs at the moment).
Certainly the Corydoras should be happier with a sand substrate, and I would leave out any treatments for the time being (some treatments can affect the bacteria in the filtration system, which can actually cause secondary problems which mask the original cause). I'm not sure whether Black Onyx is calcareous, if it is it will almost certainly buffer the pH upwards, so you could ask some of the other NY-resident list members where they got their sand from and if they were happy with it, then you can get a reliable source.
I've never yet had a sand substrate go anaerobic noticeably, but since changing over to sand I'll never use gravel again unless it's for a specialist application (and even then it won't be aquarium-bought 'pea gravel'). All my fish are far happier with a sand substrate, whether Corydoras, various Cichlids, even those that don't associate near the subtrate appear to be more settled.
I would also think about getting a small quarantine aquarium. I never introduce any new fish to an established system without quaratining them until I am content that they are not showing any problems (actual treatments are rarely needed this way, a bit of peace and quiet, good water and good food goes a long way towards helping fish settle quickly).
With the canister filters, how do you clean them? Do you clean them alternately or both at once?
Pete