I don't understand how it could have 'gone through [Marine Ich] twice.' Are you not keeping the fish healthy and disease free? Is it in a tank contaminated by Marine Ich?
A fish eating properly but loosing weight is often a sign of internal worms or parasites (assuming tank and water are perfect for it). Give the fish a dose of Praziquantel (droncit). More info on that treatment is found in this thread:
http://www.reefcentral.com/forums/showthread.php?s=&threadid=644238
The Clown Tang, similar to the Sohal Tang, has an 'attitude' that is often pushy and tough. But don't let that make you think the fish is easy or can tolerate less than perfect water quality. This tang needs a lot of swimming space (a 180 gallon tank for the juvenile would be best); top water quality; and the right environment. Like most tangs, its 'skin' is not well protected and the fish can be the target of many different kinds of parasites and even small ammonia spikes can initiate bouts of septicemia.
It's a herbivore and needs a lot of greens. It also needs a lot of food. The young ones are best fed three or four times a day since they are used to getting their food pretty much throughout the day. Of a regime of three feeds a day: one of gut loaded brine shrimp is okay (alternate between spirulina and fat gut loaded brine shrimp); a second feeding of Formula 2 (flake, pellet or frozen); a third feeding of one of a few different choices (like mini-mysis shrimp, or blood worms, or 'herbivore' prepared foods, or Formula 1, or home grown macro algae, or broccoli, or ?). This along with the daily feeding of algae on the clip is a good pattern for the juvenile Clown Tang.
Use vitamins, and fats on one feeding per day. Mix beta glucan in with the food once a day while the fish is recovering to help boost its immune system.
Good luck! :rollface: