I think with the FW dip, it seems unlikely we're dealing with flukes or gill parasites. Did you by any chance check the FW dip after the fish left to see if anything came off the fish?
Let's review a moment. At the beginning you wrote that you were sure the problem was not an internal infection. I went along with that, but now I wonder. WHY do you think the bloated fish are not having an internal infection concern? Bloating is most frequently associated with a bacterial infection. There are many bacteria/microbes in saltwater that it's hard to identify which one it might be. If it was identified, the treatment would be obvious and swift in helping the fish. But in general the internal infection swelling the abdomen are usually caused by a
streptococcus bacterium.
The fish can be treated with an antibiotic while in hyposalinity. In hyposalinity, antibiotics are more potent. So you don't want to overdose.
I would treat the tank with Maracyn Two for Saltwater fishes. This formula of antibiotic goes inside the fish, through their 'skin' and helps them with an internal infection. This formula also contains an appetite stimulant which it sounds like your CT could use right now. If it shows signs of recovery, I'd finish the 5-day Maracyn Two treatment and follow it with a Penicillin or Ampicillin treatment. So obtain both (or all three) medications for saltwater fish.
Treat with Maracyn Two for Saltwater fishes at the recommended first dosage level and then follow with doses at half the recommended amounts indicated on the medication instructions.
I think that is about all I could recommend at this point. Are you
sure there are no other symptoms or appearances that you've left out of your description?
Can you post a photo of the CT? A relatively close-up of the fish showing as much detail would be the goal.
No matter what is happening, you must maintain a close control on water quality, Lorenzo. Measure ammonia and nitrite levels at least twice a day, preferably one hour after a feeding. You should be getting zero readings on both. Take all actions necessary to provide the best water quality you can. (This is one good reason for fish to be in a hospital tank where water quality can be improved with very little notice or effort.)
I don't want to be the bearer of bad news. . .but realistically, the conditions you described are not hopeful. Unless the matter is systemic and bacterial in nature, the Maracyn Two will not improve the condition of the fish.
