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" as some people say, the carnivores pretty much just cruise around, passing up easy meals. Herbivores, such as Tangs, have a longer digestive track 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 track because it doesn't take as long to extract the protein from the flesh of another critter (snail, fish, shrimp, etc.).
Herbivores, ideally, should have a constant food source available to them. The algae that grows in your tank 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 once or more per day is closer to the way they eat in the wild than feeding them a bunch 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 2 minutes or so, then ignore the remaining food for hours. The food that remains in the tank is wasted and will end up on the bottom of your tank, creating even more nitrates.
If you only feed your 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. In my experience, feeding twice per day, only what is consumed in about 2 minutes has been the best.
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 lightly twice per day. The Marine Fish Food Comparison Chart is a good place to start if you are interested in finding the best food for your critters.
Every tank is different, but it's really not all that hard to figure out. Just watch your fish. Be one with the ocean, Grasshopper ;-)