This basic coloration crops up in juveniles of a bunch of different damselfish genera. I would need to see a closer image to have any chance at a reasonable identification. Most likely it is either a Stegastes species (which I think is what EiksFish315 suggested) or Neoglyphidodon polyacanthus ... but I am basing that more on what is likely to be in the trade than anything special I can see in your photo. One thing, is there a blue spot on the actual tail (rather than the caudal peduncle), or is that just reflection off the glass?
Tony