Hello Ring...
Well, for starters, you need to get a stable water chemistry. Even traces of nitrogen like ammonia or nitrite can kill fish fairly quickly. Change half the tank water every few days and replace it with pure, treated tap water. Add some individuals stems of floating plants like Anacharis, Hornwort and Pennywort. These take in nitrogen as a food source and take it in through their leaves. They help keep the tank water cleaner for the fish.
Cut back on feeding. Stressed and sick fish won't eat much and you don't want uneaten food to foul the tank water. Fish are small and don't need much food even though they constantly beg. Get a good quality food like the flaked type from Cobalt and feed just a little every couple of days.
Do a good job of vacuuming the bottom material, this will improve the water quality by removing decaying fish and plant material.
I would follow this routine until the fish show improvement.
B