How long are you planning to keep him in a 20 gallon long? It's hard to say if keeping him in something that small would be the right thing to do or not. It's always a possibility that he will become stressed if kept in something of that size for too long...and, as we all know, stress for reptiles can be very harmful. If you can find something any bigger than that, that is in your budget, I would opt for a larger size tank.