I wouldn't use live tadpoles either for the above reasons. The problem with using eggs is that they are covered in a jelly, this is intended to stop fish from eating them so unless you can remove it they won't be able to eat them. I have had to decapsulate frog spawn a number of times (mostly for identification or microscope studies) and it's very, very hard and usually involves preserving them first in either alcohol or formaldehyde.
I'm not just talking about the mass of foam or jelly the eggs are laid in, but each individual egg is encased in a triple layer of jelly as well, and it's about four times the diameter of the actual ovum (the black dot which the fish might get some nutritional value out of). This is the egg capsule and what comes out after it's been decapsulated is probably not the best thing to feed to a fish.