not too much offense meant here, but how is it the lfs' fault that you bought a fish that needed a tank 3 times that size? If you read even one saltwater fish book, you would have known that wouldn't work. those fish get to be 19 inches long. lfs' are usually all about making money, and while that's not ethical, and they SHOULD have told you, it still doesn't excuse your actions here. sorry if this was a little harsh, but I don't believe in "learning things the hard way"...I believe in researching for myself so I don't have to depend on an lfs to be completely truthful about things.
and I also don't like the advice of keeping a tang long enough until it can be moved. there are too many fish bought with bigger tanks in mind, and when they never get around to the bigger tank, the fish suffers.
again, I don't mean to offend, but research would have saved you a lot of money and aggravation, and that fish's life.
oh, as far as your original question, you can get by with about 13-15 inches of fish, depending on size and room requirements....meaning SMALL fish, mostly 3 inches or smaller. about the biggest you'd want to handle would be a dwarf angel.
oh and i have a niger trigger on hold up there i done payed in full for is this fish going to be to big or should i try and trade it?
