i could see keeping a yellow tang in a 15 gallon (not saying its the best thing to do) as a holding tank, maybe for a day or two, upto maybe 4 days before putting it in your display tank, but other then that, i think it would be mean to do. but what do you think about a yellow tang in a 55 long? ive read on a site that they only need a 30+ when full grown, but is it true?