I have the exact same problem. Although I had the smaller one in a tank by himself and decided to move him in with the others. He's much happier now, has gained some weight but not allot. He has grown since I got him but just not at the rate as the others. He eats all the time, I tend to overfeed...so I do more water changes. I also feed veg's, live food, pellets etc. every week. I did look up on the computer a picture of a loach with skinny disease and mine doesn't look that bad. Do you actually see him eating? I've taken the wait and see approach for now, not saying that's what you should do but as long as he's active and you're sure he's eating could he just be a runt. I have a molly that's a year old and has never grown more than an inch in size. He's not sick, very active and eats great. I'm hoping the same is true for my little clown loach.