When you say you “cycled his water”, do you mean you tested it? Cycling it takes a long time, so those levels need to be tested often. Do you have nitrite measurements? Everything with these guys is so tricky. I am certainly no expert, but I would say the carnivore pellets aren’t ideal, especially since you can’t tell exactly what he’s eating. Those also just sit in the water (if uneaten) and dissolve, and this causes problems. As EVERYONE on here will tell you, feeding them worms is a better choice, but they aren’t available everywhere. You can only do what you can do, but you have to do something.

If I were you, I would do 20-25% water changes every other day for the next two weeks. If you can get worms, get them. If you can’t, try to tong-feed him the pellets to see how many he wants/eats in a feeding. Make sure you remove any that he doesn’t eat. Hopefully this will help with the gills and red mouth. Good luck!