I keep 3 African Dwarf Frogs in my community tank and i've had no problems... I feed them frozen blood worm, which goes down a treat. I find if I feed them normal flake food they tend not to eat it.
Be sure not to confuse the African Dwarf Frog with the African Clawed frog. They are very different beasts, and the latter will grow large enough to start eating your fish in time, not because they are necessarily aggressive, but that they feed when prompted by movement (not smell) and anything that fits in it's mouth is fair game. Better to keep the clawed variety in a species tank.
I hope you mean make sure that the fish CAN'T eat them. I keep 2 ADF's in my community tank with no problem. They eat frozen foods, brine shrimp, glass worms, bloodworms etc. You have to feed them specifically, not just drop food in the tank, especially if there are a lot of fish with them. I use a turkey baster and drop the food in front of their noses, otherwise the fish tend to eat everything and there's none left for the frogs. They're very comical and ungainly - they "paddle" up to the top of the tank every now and then to get air and then dive down again.
okay the last from i had was in a community tank and i was a kid but to my best memories it ate all my FREAKING FISH sides the ones that were his size+
One of the easiest ways to tell the difference (when they are small), an African Dwarf frog has webbed front feet, and African Clawed frog has no webbing on their front feet, just clawed fingers