[leebca]

Generally speaking, the diet is deficient. If by sinking foods you mean compressed pellets, discs and things like that, then the are not good for long term feeding of marine fishes. They were made mostly for bottom and middle-level feeders in aquaculture. They contain as binding agents usually a form of wheat or wheat gluten. This is something that marine fishes don't digest.

You mention good numbers for water quality. But the feeding of wheat containing products is adding pollution to the aquarium. What the fish don't digest (e.g., wheat) goes into the aquarium and become waste for bacteria and other organisms. This raises the nutrient level of your water. There is no home test for this. So your water is also 'off' by virtue of the diet the fish are getting.

Still, there are other water conditions we don't test for -- such as organic content. Your skimmer and carbon treatments should be well maintained and removing as much excess organics as possible. In retrospect, I'd say the aquarium is getting too small for this fish. It will require as a minimum a 6 foot long aquarium before the end of this year.

Frequency of feeding should be no less than twice a day, though 3 or more is preferred. Smaller more frequent meals is what marine fish are used to and by that I don't mean just as a matter of convenience, but that is what their digestive track is build to handle. To optimally absorb nutrients from their food, it should come through the digestive track slowly and continuously.

Offer marine algae sheets (smaller but more frequent amounts) daily to this fish -- twice a day if you can.

Then look into fat and vitamin supplements for the foods. As lcstorc has mentioned, feed only whole foods (leaning towards frozens) of pods, ocean plankton, etc. even if you have to make it smaller for this fish. Keep in mind that at this size, the fish expects about 60/40 algae and pods in its diet. As it gets older, the amount of algae should increase.

Unless their are still other stressors, the fish should come around with the above adjustments.