Clown Loach - Abdominal Swelling

[Kat]

Hello,
I have a clown loach who has developed (within a couple of days) a swollen abdomen. He is clearly a different shape than the others and the swelling is even, and visible on the bottom and sides of him (top is normal). His internal organs area is visible when he is against the light, as they are darker, and this seems to be where the swelling is. He is otherwise fine, active, eating and a good colour. Any ideas? Can Clown Loaches get dropsy, and without visible scales how do you know? Is it possible he could have internal parasites?
Many thanks,
Kathryn