I ran several dart tanks with 8 inches of water in the past and never drowned an adult frog. I also ran guppies in with them. Only thing I will caution you against is strong flow. Darts are not great swimmers although I have seen them do it. A gentle flow is the max. I used a small pump that basically bubbled up through a small sponge covered in moss. This way they never ahd to worry about getting swept in or around under the water.