I think I'm on 5 now. As much surface area as the last one had, he was getting big enough to almost climb out. Or at least close enough to make him try. This one doesn't have as much surface area, but I can put 20 gallons in it for him to swim around in. That's a floating dock he's on, so I don't have to keep the water level constant. The black thing across the top of the tub is a stretchy cord; It wasn't made for 20 gallons of water, so the sides would bulge. The top right is a UVA/UVB CFL bulb. I started using it because I didn't have a place to put him in the sun, and I keep using it because when the tub is in full sunlight, the water turns green with algae and whatnot.