including frozen food, vegetables and homemade food.
hey is there any way to tell how old fish food is that on the shelf
If there isn't any "best before" date, don't buy them. Or if seller keep them in warm place (e.g. sunlight etc...), don't buy them. Unfortunately sellers keep them on shelves (in warm) and it, of course, affects on food too. I usually buy only small cans because when you open it, air fills up the container and vitamins start to destroy. And when you have opened the can, keep it in cool place: at least in your refrigerator or freezer.
If you buy larger cans, bag it in several plastic bags. Into bags that are easily opened and closed - ome plastic bags contain shutter. Squeeze air out and closed bag very good. Put those bags to freezer and when you need food, take one bag and keep it in your refrigerator... Then those vitamins last longer.
If you keep food container in warm place (e.g. on light cover), it will be only "offer" filling for hungry mouths but not any vitamins that fishes need - because they have already destroyed...
And remember too, that some fishes are carnivores, some are herbivores and some are omnivores. Feeding wrong causes so called "dropsy disease".