Number of reasons, but mostly costs. When you 'hyper-stock' (can I coin the term?) there's so many fish, mostly through mass confusion, it's impossible to set-up a territory and this prevents fights. The tangs, or any fish for that matter, are stressed, but it would be ridiculous to keep 30 30gal tanks at a fish store for each tang. They're obviously not permenant tanks so the 6 foot rules or whatever don't apply.