carnivorous fish will do great on live foods only, like bettas. Mine are only fed frozen bloodworms, brine shrimp and mosquitoe larvae.
but omnivorous fish like guppies might become ill or become weak if there is no vegetable matter in their diet. In fact, they require more veggies than meat
Feeding only on live food, when the fish you're talking about is not a carnivor (so is an omnivor or herbivor) it will be bad for them. They'll get too much protein and this is likely to cause bloated. It will also weaken the fish, make it more likely to catch other diseases.
Yes, I agree with those posted. I assumed in my post the fish in question was a carnivore.
If you have omnivore, say, platies or guppies, you can feed a lot of vegetable matter and some live food. The problem with feeding mostly vegetables is that most of the veggies we buy are land-based and have different nutrient makeups than aquatic plants.
For omni/herbivores, it might be too difficult to feed fresh food, but carnivores is OK.
Obviuosly if your fish won't eat flakes, then you don't have much choice. O'm feeding Vespasian (my betta) pretty much only on brine shrimp, however when I go to the shop next I'll try somethig else.