Blue Danio disease

Hi. I have a few Danios in my community tank. Some of there are a bit overweight as they always eat first, They follow me around the tank as they now I am the top of the food chain.

I had to take one out as it could not swim anymore and put him in a small holding tank.

First question. What diseases do they get? He is just fat, not bend in any way and nothing crowing on him as I can see. He is in a holding tank now for a week and slowly getting thinner, but still can swim. I still feed him but very little. What disease if any do they get?

Last question. How do I know if I feed my fish to much or too little as they always look hungry? The Danios and Plattys are always first to grab the food when I feed them so they are on the fat side. The rest of the fish (Black widow Tetras, Congo Tetras, Neons and Cardinals) look "normal". The ground feeders are fed sinking pellets but the Danios nibble on that as well.

Any suggestions?

Thanks