I was thinking about a frog also...mine doesn't move around all that much, and I have him in a two gallon with a betta for the time being, and it is working wonderfully. Also, how bout not really using it for fish, and growing some earthworms or something, you could feed them to your fish depending on what you have...or grow some and stick them in the garden when they are big enough....how bout an antfarm? well, those are probably not the kind of answer you're looking for, but hey, I had to try, right?