PlanetCatfish.com

I've not had much luck with Corries in my Community Tank recently. In the last month the 3 Duplicareus I bought last year have died, also 2 of the 3 Trilineatus that I bought last month. I still have my original 2 Melini which I've had for over 2 years now, but can't seem to keep the newer ones. So I've been asking friends about this, one said that he was told by a breeder of Corydoras that they should never be kept in a tank with CO2. I added a unit about 3 month ago, and was wondering if this is causing the deaths. I've never heard this before, is it true? I'd be interested in your comments.

The Melini are big guys and I also have 2 bigginsh Zebra Loaches, so food isn't a problem, but the Dupliacaeus never seemed to grow very much in the 10 - 11 months I had them, is this normal? Also the one remaining Trilineatus spends most of his time swimming mid tank, and when he rests he seems to be on top of a stone wall and not on the ground. He does spend some time on the ground though.

I also have 4 Pandas in another tank (without CO2) and these little guys seem to be doing OK

It occurs to me that if your loach is doing alright with the CO2 injection then so should the Corys.

Some live worms are very good foods for getting new Corys back in shape after you buy them. Their plated bodies tend to mask the normal signs of prolonged starvation. I guess you can see where I'm going with this. Corys often die after being moved that one last time from the vendor's tanks to the home aquarium.
They are often not fed for weeks as they make their way through the distribution chain.

If the fish had died shortly after being added to a CO2 tank then I would suspect it could have something to do with it (pH change etc), but you say you've had them since last year so that doesn't sound like it is problem.

Also corys will take air from the surface when the oxygen levels in the water are low, I imagine they would also do this is the CO2 was on the high side. If they had been doing this more frequently it could indicate the CO2 is distressing them.

I am aware of a few planted CO2 tanks that have had corys & hoplos in them for several years without any issues, so I think the cause may lie elsewhere.

fwiw, what is the pH & kH of the tank and do you regularly add any buffers?