Easy to answer. Many of us do care about what is going into our fish, and all fish foods are not the same. Many contain stuff that the fish should not consume, like cereal meals. Fish meals are not as nutritious as whole fish, and you can buy foods with the latter and not the former so these are healthier. As in all animals, what they eat determines many facets of their health, behaviour and well-being.
The #1 food for all fish that are carnivore or omnivore is Bug Bites. Fluval make these, they are whole soldier fly larvae with nutrients added. Most of the fish (carnivore and omnivore) eat primarily insects and insect larvae in their habitats. Even Corydoras catfish. The second most common food are crustaceans, and here shrimp is good. Frozen shrimp is fine, and frozen daphnia is excellent, all fish will gobble these up. Worms should be minimal, once a week, and only frozen or live. Herbivores need different foods containing "greens" like algae, kelp, veggies. Here I would feeed Omega One Veggie Flakes (upper fish) and Veggie Rounds (substrate fish). Corydoras cannot digest plant matter, so do not not feed these to them; once a week is OK, if there are herbivores like otso, Farlowella in the tank. If you have vegetarians in the tank, fresh veggies (usually blanched first) like cucumber, squash, greens may be taken, just make sure they do not foul the water.
As for feeding, many overfeed and this is not good. Not only does it make the fish susceptible to other problems, it can affect the water quality. Certainly no more than once a day, preferably in the early part of the day (day meaning the period that the tank light is on), and never right after a water change. The fish need to be "settled" after events like water changes, tank light coming on, etc. Do not overfeed. Upper fish should be finished in a few seconds, no more than half a minute. Substrate fish need more time, usually a couple of hours, to nibble on sinking disks, pellets, etc.
