I wouldn't. You really want to be able to easily tear the QT
down and clean/disinfect it. I would probably just have a decent HOB
filter and perhaps a couple of pieces of PVC
pipe for the inhabitant to hide in. You'll have to be very careful using this tank even as a QT
due to the small volume of water. Things can go wrong very quickly in a 5g tank. I'm not trying to discourage you though...a QT
tank is very important and, as long as you check the water parameters every day, I think you'll be fine with it.