I feed my fish and then change the water
Days go by before they are fed again.

Hah,that's a good one!I wait 24hrs to feed only to let the fish enjoy the water quality.
catfish is oily? Uhm, catfish is classed as a delicate - a white meat, along w/ others like bass, cod, hake, burbot to name a few....... oily darks would be more like tuna, swordfish, bluefish, salmon etc. Your medium textures are fish like trout, bluegill, carp, and basically anthing that doesn't fit under a white or a dark, but somewhere in betweenI frequently feed messy food the night before a water change, like catfish, which is oily.
I then do a water change usually earlier in the day, and typically feed in the evening... if they got normal food the night before they get fed that evening. If they got live food or frozen fish they fast a day.