His scales are normal so it's not dropsy. Though, his gills aren't closing all the way much. It's only by a little bit, but they're differently not closing.
Right now, I have him in a separate, smaller, and much less decorated tank. I put in some stress coat as well as aquarium salt (by the way, is it okay if I put aquarium salt and Epsom salt in at the same time? Or is that dangerous for the fish or something?) and I am planning on doing 100% daily water changes for a couple days to see if that will help.
And thank you for telling me about the ammonia thing. I actually had no idea! Some fish owner I am. :P
Thank you so much!