More info:
I'm planning to upgrade the 10 gallon my current ADFs are in. The space for the tank is very limited, so I could only upgrade to either a 15 gallon long or a 20 gallon high. I'd be keeping everything from the old tank including the filter and light, but likely adding an additonal adjustable filter. But the aquarium itself and the lid would be new. The temperature would be 78, the hardness level 11 dGH, the pH 7.4. Temperature could be lowered some if needed, though. I'm going to try to keep the current slow.
This will sound weird, but I'm actually looking for an upper water fish to eat tadpoles. I've had 2 tadpoles in this tank turn successfully turn into adult frogs (out of 2 tadpoles) and just now found 9 more tadpoles. I know ADFs are supposed to be super hard to raise successfully, but the tank conditions in my setup seem to be just right for them to make it. The adults hang out mostly at the bottom, and don't seem to really notice the tadpoles that always stick around the surface of the water. So the adults aren't eating them.
I'm hoping to hear ideas for tankmates that a) would eat tadpoles or fish fry, but not bother the adult frogs b) would swim mostly in the upper areas of the tank c) wouldn't harass the frogs, be clamped onto by the frogs, or outcompete them for food if I target feed d) could live as a single fish or at most in a trio, so that most of the bioload could be the frogs.
I'm hoping to thoroughly research everything while the tadpoles are growing (or not), then upgrade the tank, make sure it's fully cycled, and then only try the tank mate if some of this newest batch of tadpoles turn into frogs. But I figure even if I don't end up needing the fry eater, it wouldn't hurt to research it while I wait to see what happens with this new group of tadpoles.
From what you've said, Bettas are definitely out. But what about a single Dwarf Gourami or a trio of Sparkling Gourami? Or Guppies? When I was young I had a Guppy colony and my Mom for some reason added a feeder Goldfish. The guppies went crazy biting chunks out of the Goldfish. That was an image I'll never forget. Would the Guppies also nip the African Dwarf Frogs? A trio of male Guppies would otherwise fit what I'm looking for, but I'm especially worried about them nipping the frogs. Has anyone kept them together and ever seen them nip the frogs, or will they leave them alone? I'm hoping to find other fish species to research I haven't thought of yet, too.