The key to predators in a small tank is nutrient export. I obviously used a bare bottom, which allowed me to siphon the heck out of it and then do sizeable water changes weekly, but plumbed into a larger system as longa s you can keep the detritus in suspension, your main filtration will take care of it. for reference, my frog was about the size of a ping pong ball, you can see him sitting on the maxijet. If you can, get him to eat from a stick... there are threads on how to train them to do that. Mine never would, so I had to set up a deal with a local store to get bulk chromis (i went through about 12 a month) but was able to get em for about 1.50 each as I was specifically culling the tiny ones out of his shipment.
while you will read that it can be done, I don't recommend mollies or guppies. I lost a beautiful black angler to a mysterious fungus that I still attribute to dirty fresh water fish as the main part of his diet.