How did you get him started? Got mine a couple of weeks ago and near as I can tell, he only eats algae.
It took mine almost 6 months to really start getting an appetite, upon the intital purchase he was really underweight.
I took at as a personal mission to save the little guy/gal.
I started first with shredding omega one veggie sheets "green" into small pieces and I mean small, as this was the only thing that I could get my clown to keep down. I feed him almost 5 times a day for almost a month. Slowly he would eat more and more.
I also kept a veggie clip on the tank with the seaweed that he could actively graze on throughout the day which allowed the fish to slowly gain weight and I mean it was slow.
Continuing on I would soak the shredded pieces of seaweed in mysis brine and after a couple of times of doing this my fish really started responding well. I then started feeding mysis to him [note he would not eat it before] and he started chowing down! I saw a good amount of weight gain with this method 2x a day!
It took almost 5 months before actively eating flake and mysis! Now he eats absolutely everything and anything that I put into the tank! My clown actually out feeds all the other fish in the tank! And I'm proude to say it's one of my most healthy fish!
It takes a lot of nurture to get them to start eating it can honestly be down right frustrating at times, but be patient while looking for signs of stress and infection. Other than that this is what I did and was able to get my clown tang fat and healthy! Totally worth the pain in the a**
Hope that helps! :)