You can try Prazipro to treat the clowns. It's a very safe medication. I've used it a few times and it might have helped when I used it.
IMO, older larger wild caught clowns do not acclimate well to the captivity of aquariums.
I got 5- 6"+ wild caught clowns '1.5 YEARS' ago that would't eat. 2 of them I had to euthenize about 4-5 months after I got them because they got very skinny and wasted away. I might have to euthenize 1 more soon because he's getting very skinny. Only 1 of them has been eating staple foods regularly and is fat and healthy. The other skinny one has just started eating cucumbers and might have a chance.
Here's a few thread's I started about these clowns-
http://forums.loaches.com/viewtopic.php?t=534
http://forums.loaches.com/viewtopic.php?t=3866
I've been feeding the skinny clowns shrimp, clams, and snails mostly.
They are missing something from nature that is essential in to diet, but I don't know what. Perhaps it's live fish. After I introduced these clowns to my main tanks after a quarantine period, about 50 tetras mysteriously disappeared.
I hope your big clowns the best.