Your fish is sick from overcrowding and the poor water quality that results from it. One four-inch fish is way too big for a whiskey barrel. For four fish, you should have at least a 100 gallon pond.

The size of habitat might be similar to what my 7 goldfish experience which I keep in 40 gal of water in an aquarium in my basement over winter. They are healthy and very active. Half days I have fluorescent light, a bubbler going at 60 dgr.F. We feed them 20 goldfish pellets a day. Every 4 weeks I vacuum out the gravel, wash it, and exchange 25% of the water with clean water. The water is faucet water which I let sit in pales for one day/night to let chlorine gas escape. I have done this every winter for many years.