In the wild, looking for food is one of the three things that saltwater fish spend their time doing (the other two are reproducing and staying alive by not being eaten by a bigger fish).
Feeding Your Fish
If you watch saltwater fish for a while, especially in the wild, you will notice that the herbivores are constantly looking for food or "grazing," while the carnivores just cruise around, looking for the next easy meal. Herbivores, such as Tangs, have a longer digestive tract than carnivores. This is because it takes longer to digest algae in order to extract the protein required for the fish to survive. Carnivores have a shorter digestive tract because it doesn't take as long to extract the protein from the flesh of another critter (snail, fish, shrimp, etc.) to keep them healthy.
Herbivores, ideally, should have a constant food source available to them. The algae that grow in your aquarium will provide a certain amount of this, but unless you have a lot of it, you will need to supplement their food supply. Prepared foods, such as many flake foods, are concentrated food and (depending on the type) can supply the fish with everything they need. Feeding your fish a little bit of food several times per day is closer to the way they eat in the wild than feeding them a bunch of food every 2 or 3 days.
Most fish (even sharks) will only eat what they need to survive. If you watch your fish when you feed them, you will see that they will actively eat for a few minutes, then ignore the remaining food for hours. The food that remains in the aquarium is wasted and will end up on the bottom, creating toxins as it decays.
If you only feed your herbivorous fish once every 2 or 3 days, it is not the way most fish eat in the wild, which is what we should try to duplicate as closely as possible. In my experience, feeding twice per day, only what is consumed in about 3-5 minutes has been the best. This gives the fish what they need to remain healthy and at the same time doesn't create any maintenance problems in the future, such as more water changes to reduce nitrate created from uneaten food left on the bottom of the aquarium.
Carnivores, on the other hand, are a different thing. Eels are an excellent example. They will go for days without eating, then suddenly feed voraciously for a couple of minutes. It has been my experience that if you have a mix of carnivores, omnivores, and herbivores in your tank, they will all find and eat what they need if you feed a variety of food twice per day.
What Food to Feed
If you examine the contents of the commercially available saltwater fish foods, you will find that there are three basic formulas: those containing mostly algae, those containing mostly seafood such as shrimp, krill, mussels, squid and various fish, and those that contains a combination of the first two. There are also a number of frozen fares available which are excellent sources of protein.
We have found that most herbivores will consume flake or pelleted food made with algae and other ingredients. Dried Nori algae sheets are also available to use to feed herbivores. Herbivores will also pick at many of the frozen fish foods, such as Mysis and brine shrimp.
Some carnivorous fish will eat meat based flakes and pellets, but others might only eat real meats, such as brine shrimp or chopped fish and krill. Live food such as ghost shrimp, guppies or mollies are also fed to some predatory species of fish. We have also found that new arrivals to an aquarium pick up on eating prepared foods as they see other fish eating it.Most fish species will learn to eat prepared fish food.