Size of the tank in my experience, which is considerable with Tangs, doesn't matter as much as rock and FLOW.
Tangs need a place to swim and a place to hide.
Unintentionally at first, I kept a YTB in a 25 successfully for 4years. Bought about 1-1/2 inches with plans to upgrade to a 110 at the time... well those plans kept getting pushed back, but I kept that tank with rock, all except the very front, so I would need to buy less when time for the upgrade finally came. I did however put a MJ 900 in the front and halfway down, creating a swimming lane for him. And he loved it!
Seriously, occasionally I would forget to turn that PH back on after cleaning/feeding, etc, and he would go over to the PH and thump the glass with his body/tail until I would turn it back on. And the only time he ever got sick was when it was off for several hours at a time.
He loved his swim machine. And he loved all the rock. Put him in a 75 with a normal amount of rock for that tank--80 pounds--and he hid for weeks... until I made a PH give him a swim lane again.
I've applied the same idea to tangs we've had in the store not doing as well at first.... give them more places to hide and more current to swim in and they do better--like clock work.
Finally, IMO, the best way to be successful with a small tank and tangs is to start with a realy small tang--I imagine they miss the great open blue a lot less because they hardly swim openly in it til they get bigger.
It can definitely be done in a healthy way.... and yes, obviously at some point their growth would dictate a larger tank.
