I would doubt that toad tadpoles would be good food for fish. We use to have a pond in our back yard that had healthy populations of several species of frog and American toads. It was a temporal pond that dried up in August so had no fish. After several years we deepened the pond so that it wouldn't dry up. After stocking sunfish, bullheads and several types of minnow the frogs stopped breeding there (except for bull frogs). The toads did just fine however. Every spring hundreds would come together and literally thousands of little toadlets would leave the water. The fish wouldn't bother them at all. (one year a huge snapping turtle enjoyed eating the eggs but that's another story) So I would suspect that toad polliwogs aren't all that palatable to fish. Also having raised many to toadlet stage, I can say that at least toad polliwogs are mainly vegetarian. That is pretty much all they would eat unless another polliwog died of natural causes. Then they would go after that as well, but it was the exception.