Hi and welcome, it is nice of you to take care of the fish.
As it was stated above we need all this info to help you, however it does sound like Swim Bladder Disease. I will let you read up on this, in the meantime fasting the fish (all of them it will be good for the others too) for a couple days and then feed thawed deshelled peas cut to bite size pieces. Only 1 pea per gf., for a couple days should help.
I will also say that having 4 gf in a 20 gal tank is way overstocked, so I would bet the water quality isn't very good. FYI just because it looks clean doesn't mean it is. GF are messy, massive waste producers and living it that can hurt them or even kill them so testing the water is a must.
You can let us know the results and we can help you further.