I'm fairly certain dart frogs wont eat fish? Correct me if I'm wrong . My main concern would be the amount of water you plan on having. There are very few fish that can survive in less than 5 gallons of water and pretty much none that can thrive in less than 10 gallons (except bettas, but they still need at least 5 gallons IMO). So if you wanted to have fish you would need a decent amount of water. I would assume a set up like what you're talking about would have to be in at least a 40 breeder. Now, I don't have any experience with dart frogs or with building tanks with water features. I'm just speculating using my knowledge of fish keeping. I'm sure someone else who has more specific experience will chime in too