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Hi urchysj,

I think the problem occured due to a combination of factors. The frozen food may have been off as you noticed it had a different colour and it may have been left to thaw for too long. More than likely too much food was given but I can understand the reasons for this. One of the external canister filters was switched off, this was probably the main cotributing factor. I think that all of these factors combined fouled your water and resulted in the fish wipe out.

You seem to have identified most of the problems already and possible remedies. A speices only tank is a good idea and would prevent the temptation to overfeed so as to ensure everyone gets a fair share. Plecs are generally not able to compete effectively for food with other "greedy" community fish.

A clear indication that your water had become fouled was that the large water change almost immediately cured the problem.

DAN,
I encountered the same exact problem with a planted tank a couple of times.

As soon as I read your post I wondered if you use CO2 - since you do I am almost certain the accident is due to a CO2 malfunction.

When a cannister is almost empty it seems to discharge the CO2 faster for some reason also there are a multitude of ways the gas can escape or trickle in.

Reason 1 - if the gas is on all the time the valve(s) may have worked themselves open.

2 - if you have a solenoid , it can malfunction. If there is any electrical connections to the system there may have been a poweroutage or surge to discharge excess CO2.

I have had plenty of tanks with waterlevels all the way to the glass without problems , your description of the aftermath sounds like typical CO2 poisoning - sorry for your loss.

Even the great Takashi Amano has had his problems with CO2 poisoning - he mentions one of his tanks getting wiped out by it in one of his books.