The live fish food Revolt-O-Meter goes off the scale!

“Can I feed (earthworms, flies, mosquitoes, aphids, etc.) that I found in the yard to my fish?”
Yes, but do bear in mind that when you feed wild critters to your fish, you have no way of knowing whether they have ever been exposed to pesticides or herbicides that may be toxic to your fish. If they have, and your fish get poisoned, don’t say I didn’t warn you. Aquatic insects, worms and larvae that come from environments where fish live could potentially carry disease into your tank.

“I have ants in the house. Can I feed them to my fish?”

Maybe, depends on the fish and the ants. All ants have honkin’ mandibles and some are capable of delivering a nasty (possibly even venomous) bite. Some fish are little. Some fish are big. You decide whether it’s worth the risk.

“I have (fruit flies on the bananas in the kitchen, fungus gnats in my potted plants). Can I catch them and feed them to my fish?”
If you do not use insecticides in the house, yes. Can you catch them? Well, good luck with that!

“What about gnats around the cat’s litter box?”
Uh…

First of all, you do know it’s not normal to have flies buzzing around the ol’ cat box, don’t you? Maybe a housefly that wandered in, but not flies as in plural, and certainly not enough or often enough that you consider using them as fish food. I’m going to guess they’re either drain flies or phorid flies, both of which breed in damp, decaying organic matter. If “damp, decaying organic matter” describes anything you have in your house, it may be time to consider doing a little cleaning. Probably wouldn’t be a bad idea to check and make sure the cat is even using the box. Cats are fastidious like that, you know.
Jeezus!