EscVelocity
Aquarium Advice Activist
Corries don't tolerate salt well at all, and can stress them. It can kill them.
Thanks dkpate, I hadn't considered looking for a disease but I'll be sure to read up on those tonight when I have some free time.
To answer your questions Blueiz, I do rely on the plants to oxygenate and while the other fish in the tank are all active and not gasping for air, perhaps the oxygen is just low enough to upset the corries? I could adjust my cannister output to disrupt the surface just a little bit to experiment. Yes, there are lots of shady spots and the corries do tend to congrate in them during the day.
Tank mates are 3 small marigold swords, ten neons, two small SAE's, a mess of fry and snails, and four black mollies. none of the fish are in any way aggresive, though the black mollies are very aggresive eaters. The second they inhale all the flake food they dive down and go after all the catfish wafers. I think I overfeed a bit to ensure the other fish get a chance, which is probably why my snail population is getting out of control.
I'm no expert, but the fish seemed healthy enough when they entered my tank. I acclimate them with the bag method but no, sadly a QT tank is not yet in the budget.