Obviously, if we're talking show examples of tangs (7" inch examples) a 10-20 gal quarantine is going to be too small... but for the most part, average fish from 1 inch to about 4-5 inches will be fine in the smaller QT -- assuming you're doing the upkeep to maintain the tank as described earlier in the thread

If you were to keep that 29 gal around, it'd be a great place to QT most anything you'd need to individually..

-Corruption