I have a 12x12x18 and my frog is stedily growing out of it. If you can wait, save for at least an 18x18x18 though 18x18x24 is the best for whites from what I've read. I use UTH's and one keeps my smaller tank (12x12x12, roughly 5.5 gal) at a nice constant temp with a slight variation from one side to the other (the UTH only covers half the bottom). The one on my larger tank for my whites dosn't quite warm the tank all the way to the top, though. I have a second small UTH that I put on the side towards the top of the tank if the temps start to drop too low.