OK, so it's not any kind of parasite brought in with new fish, it's not the usual water quality problems, and it's affecting multiple individuals. Unless you've radically changed something in the last few days (fertilizers for the plants? adding (or adding more) CO2? bump the thermostat up/down? etc?), this sounds like some kind of acute poisoning. You say you added these same plants to your other tanks, with no ill effects to the inhabitants. Go double check that real quick. If you see anything funny, I'd pull all the plants right away. If you don't see anything odd, do a BIG water change in the pleco's tank — maybe 75%. I might still pull the plants in the pleco's tank, just to be safe. Then sit back and hope. Do you have a quarantine tank? Perhaps you could put the sick fish in there. I would suggest transferring them to another tank, but I wouldn't want to risk exposing healthy fish just in case. Can you think of ANY substances that might have gotten into the tank (soap, perfume, deodorant, etc)?
