[JanS]

Swim bladder is possible, since they don't always look like the textbook examples, and loaches don't have scales which is a helpful diagnostic clue. How is he swimming? Usually with swimbladder, they have a hard time swimming normally.

Another possibility is internal parasites. That would be treatable with a deworming recipe that you can use with food.

The last possibility is an internal tumor, which you can't do much about.