Thank you everyone.
I set up the tank in 2000 and bought 2 Danios. Years ago, the other Danio did a Free Willy during the night from a holding tank I use when cleaning the big tank.
This guy has seen many Angels, Dalmatian Mollies, Strawberry, blueberry & lemon tetras come and go and now it's just him and an 8 Y.O. Pleco. One has to wonder if its swimming The Green Mile.
I've been wanting to take down the tank now for the last few years and haven't purchased any new fish. But I am not going to do this until both the Danio and Pleco are gone.
I turned the heater down after an hour on a high setting then this morning, he was back swimming around. Now he's just lounging quietly in the corner.
Just hope the heat along with the Melafix and Stress Zyme will sort him out.