Kinda, sorta...
For many fish (just like kids) what they will eat is not always what's good for them. Plecs will eat dead fish and prawns quite happily, but neither should be a staple, and fed too often can lead to problems with their internal organs getting covered in excess fat. Puffers will eat feeder fish if you provide them, but fish don't do anything to wear down their teeth, and the best diet for most puffers contains mostly snails and crustaceans. Catfish will eat bits of chicken or beef, but the fat in the meat clogs up their internal organs and can cause harm over the long term. Angelfish have evolved to eat almost exclusively mosquito larvae and other aquatic insects, so while guppy fry are probably a perfectly serviceable treat, the best balance of nutrients will come from frozen bloodworms and the like.
All in all, with any fish, you should feed fish according to the following:
[1] Flake or pellets. These have the 100% best balance of nutrients. If your fish will eat them, you don't need anything else, and you needn't worry about vitamin or mineral deficiency.
[2] What they eat in the wild. Give them what they would catch for themselves, and you can't really go far wrong. Bear in mind though that a pike cichlid isn't catching goldfish in the Amazon, and surprise surprise, goldfish aren't a good diet for this fish. Frozen alternatives are usually cheaper and safer.
[3] Experiment and improvise, but only as a treat. There's all kinds of stuff that can be safe. Woodlice, spiders, flies, and so on you find in the house will be happily eaten by many fish. Snails are enjoyed by many fish if you crush their shells. Seafood and fish meat is always worth trying, and fruit and vegetables shouldn't be ignored either. Try a little and see how your fish react. Ask about and see if other aquarists have used these items. Obviously avoid potential hazards such as pesticide and herbicide residues: if you can't be sure the food is 'clean', don't use it.
Cheers, Neale
Most fish will eat anything that fits into their mouth..