Hello Lady...
If your fish has the "ich" parasite and was in the main tank, you need to put the fish back and start removing and replacing half the tank water every 2 to 3 days.
Add a bit of standard aquarium salt to the new, treated tap water. A teaspoon for every 5 gallons of the new water.
Turn the lights off in the tank and in the room too and increase the water temp to 80 degrees.
Reduce the feeding to a little, 2 times a week. Keep this up for a minimum of a couple of weeks or until you see improvement in the fishes' health.
B