lol the actual fish won't get that big, but even at 7" from top to bottom (as the ones where I last worked were) it's nice for the fish to be able to move vertically as well as horizontally! Maybe 18" is a little exessive but as the price is not that much higher and the lighting requirements are not realy much more it seems a little silly to go for something 12" tall when a taller tank can only be of benifit.