It could be HLLE
, which does not have any absolute causes, but a lot of fairly good speculation. Its usually due to stress ( which this Tang will always have due to being in a smaller than recommended tank), poor water conditions (how are your parameters and how frequently do you do water changes ?), poor nutrition (sounds like you are ok here), stray voltage (not sure for you) and one study showed that using carbon filtration lead to this (something about it filtering out something that Tangs need).
Also, you mentioned no live rock? Hopefully you have some great mechanical filtration and you are on top of cleaning that every few days (weekly at worst), because just having sand for BB will for ably not cut it for biological filtration.