Okay, I know this has been addressed several times, but people please research before you buy a fish! A 38 is too small. I know people say that they have kept them in a small tank and they have survived, but that is playing russian roulet with your animals, not really fair to them. It is stressed! All of your water parameters should be at zero! Ph should be atleast 8.1, and basically he needs a lot more room to be able to swim. They get stressed when they can't swim around. They need at least a 100 gallon tank. Here, don't listen to me, read what the experts say.
Yellow Tang, Hawaii - Zebrasoma flavescens
Also known as: Yellow, Hawaii, Yellow Sailfin Tang
The Zebrasoma flavescens grows up to 8 inches. The Yellow Tang, Hawaii prefers a tank of at least 100 gallons with plenty of places to hide & swim. The Zebrasoma flavescens is an herbivore and likes to eat marine algae, zucchini, brocoli, mysid shrimp, Spirulina, lettuce, seaweed, sponges, and other meaty treats. The Yellow Tang, Hawaii is a medium maintenance fish and may act semi-aggressively toward other fish, but aggressively towards other Zebrasoma species of Tangs. Reef-safe. Keep water quality high (SG 1.020 - 1.025, pH 8.1 - 8.4, Temp. 72 - 78° F). The Yellow Tang, Hawaii is commonly collected from Hawaii.
Please try to get him in a larger tank. It isn't his size, it's what species he is!