Im surprised it is only 1 fish that shows an issue...
50% once a week with 3 feedings a day and 4 fish in a small 20 gallon is a recipe for disaster my friend... The water changes you are doing aren't even sufficient for daily changes let alone weekly!
You want to start doing 75%-100% daily and make sure you get all uneaten food out. If you are adding pellets and they dont eat them and they sit there for a week you are basically poisoning them.
What is your plan with them? Are they going to a bigger tank with other fish...
I'd start with big daily water changes maybe even twice a day and make sure the temp stays stable. Sounds like there may be some bloat going on so you could add some epsom or aquarium salt to try and help that fish pass the blockage