I currently have a naso tang, about an inch and change in my 60g cube. He is of course going to be moved to the 180g as soon as he is big enough to fend for himself. I would put him the 30g until you find a new home for him... Just sayin the listing that you are looking for a so and so size tank for him to go into. Don;t just let any tom dick or harry come along and take him. lol That would be the best solution all around. Even in a 90g he would outgrow it before you know it.
It's nice to see people actually show thought and consideration with their fish, so many do not. Thats what gets the "tang police" so bent out of shape sometimes. hehe
Do what you can for now though, provide plenty of food and greens, put out an algae clip with some strips on it, keep him as healthy as you can while you have him.