I noticed about an hour ago that Poseidon was on his side so I looked and sure enough, his stomach is big enough to where the scales are a little raised from the swelling.
I removed him from the tank because he was in with Mercury and I know SBD isn't contagious, but I didn't want to take any chances. So now he's in my 1G hospital tank and I lowered the water level a lot. I also added some aquarium salt. I've read that there's no cure, but is there anything I could try? I'm going to fast him for a couple days and see if it goes down, but I think it's past the bloated stage.