I have one in my 20g. It's a cute fish, but EXTREMELY tiny.
They're also relatively expensive and short-lived. Well, it's more the short-lived part that puts me off them more long-term. They're usually about $30 when you can find them, which isn't bad, but they only live a couple years at most. And you really want a few of them to make a school. So you're talking something like $150 to have a good chunk of them in your tank, but then you have the second problem.
Because they are so tiny, they look very appetizing to anything larger. So if you have larger fish that have a willingness to eat fish... it's just a matter of time until one of those fish gets a very expensive meal. And on top of that, they're small enough that a large aiptasia could eat one of them without issue.
Short version, I think they're best suited to a smaller tank where none of the concerns are an issue. Great fish for 2-10g, less of a good fit for a larger tank. I honestly think even my 20g tank is too big for the one I have, at least as it's currently set up. They'd be good as the only fish in a tank, especially if you can set up a breeding population so that you always have replacements for the fish that age out.
