depends on the corals.... most corals are carnivorous in nature few are planktivores. LPS and larger palythoa can generally eat larger meaty foods such as mysis shrimp finely chopped fishmeats clams etc. sps tend to go more for smaller micron foods like cyclops, oyster eggs etc, spot feedibng is a good route to go with most lps and palys as it prevents excess food drifting around your tank rotting. I would avoid warm shelf products such as "marine snow" from two little fishies, as well as warm shelf phytos, most of them are dead and of little utility to reefers other than as pollution. there are a bunch of diy fish and coral food threads on this site that can give you some great recipies for food for your tank. if you want to feed a phyto, fresh live phyto is the way to go. this will boost pod populations which in turn feeds your corals and fishes. in such a small tank as a 20gallon I would be cautios about over feeding. lots of your corals will actually get nutrition from the foods you feed your fish oncce the fish passes the food as feces, as most fish only remove about 11 to 15% of nutrition from food before they pass it. so the feces is still full of nutrients.