IMO at 5 inches it should be in at least a 55 gallon. However i think GT should be housed in 75 gallons or bigger. They are slow growers but they still need their space. In a 30 gallon you might be able to get away with it for a couple of months but your going to do big water changes weekly just to keep the water clean. I had a friend who had a 4-5 in GT. His 55 gallon tank sprung a leak and he had to house him in a 30 gallon for about a month. He was doing two 30% water changes a week to keep up on his water quallity. 