What could cause a fish to be very lethargic? I have a couple of corys in QT and I actually thought one was dead yesterday. It was not moving, just sitting on the bottom of the tank, tilted to one side a bit. Water parameters are good. I bought a few corys about a month ago, had them in QT for about 3 weeks and within a week or two of being in my main tank I lost 2 of them (and I really don't think it is a problem with the main tank). I'm beginning to wonder if the store is just getting in some bad batches.