Thats A LOT! i thought 4 would be most since they grow 6-8 inch. I'm only planning to add one more, the big reason i want to is because my other 2 always hide but one comes out sometimes after feeding time (he isnt afraid of me). If the other one comes out (he's full grown at 6 inch) and sees me he would go back and hide, zipping through the water at high speed. Im thinking if i get another, they all might come out more and the big one i have wont be as skittish any more.