The lids on them don't seem very secure... how many gallons is the biggest one? You can start with a 10gal if you really need to, for a baby ball, but you're going to have to upsize QUICK. 30gal seems to be the minimum recommended size for adults, and I don't think they make Critter Keepers quite that big. So I guess it's up to you if you want to spend the money, only to have to spend more money on a bigger setup in the next few months...