Depends on the size of the fish you will be qt'ing in my opinion. I happen to have a 55g sw
tank still setup so I;m using that right now. I plan on using a 29g I will be getting for free from a relative.
Alot of people use a 10g though, I don't think i would qt a tang in a 10g though unless he was tiny or anything else that really big.
On the plus side, you pretty much just need a tank, heater and a filter. No sand or rock is necessary. You can add some pvc
joints for the fish to hide and be safe in though. You dont even need a light lol.
You want to keep it as bare as possible so that if you do have to treat the fish for something you arent soiling any live rock or sand as they can absorb any medications you add.
If my blue tang in qt does show any signs of illness i will be moving him to a 10g tank for medicating. Luckily the tang isnt that large.