DavidK,
I'll give my personal experience. I have 6 tiger barbs, 6 cherry barbs, 2 Oto's, and 2 cory's in a 20gallon tank. I have a big bag of staple tropical fish flake food that I mash up into very small pieces with my finger. Since my tiger's are very aggresive feeders, they will eat almost everything that floats before the less aggressive fish get to it. I use a small plastic cup and dump half of the crushed flake food into it, then add some tank water. I'll then swish the cup around so that the flake food is wetted, and then when I add the dry flake food to the top (for the tigers), I quickly pour the wetted flake food into the tank away from the filter. What this does is gives multiple levels of flake food so everyone gets some to eat. This is the staple food I feed 2-3 times a day.
I also recently bought 3 more types of food and use them normally once a day or once every other day. The algae wafers I talk about later. I bought these sinking shrimp pellets, that look like rabbit food. The fish really don't like these and I'm forced to watch them rot at the bottom, or vac them out. I would probably not recommend getting these..
I bought a container of freeze dried bloodworms. These are without a doubt the most desirable food I feed the fish. They should not be used exclusively as the only food, but they are very high in protein and all the fish love them (except the tigers get most of them). I do the same technique with the bloodworms as I do with the flake food to try to get some to sink for the other fish.
KhanJee,
For cory's (I have 2), they absolutely LOVE algae wafers. These are sinking green wafers that will give them a great balanced diet. Since they are most likely malnourished, I would feed the algae wafer at the SAME TIME that you feed the flakes. This will hopefully keep the other fish at bay (they will also snack on the wafers) long enough for the cory's to get a bite or two. These also degrade slowly and I find that after a couple of hours, small pieces can be found on the bottom that the fish don't eat, but that the cory's when rummaging for food, will eat.
Another thing you can do is put fresh slices of cucumber or a small stalk of partially boiled broccoli. I use a mag float (or if you have a vegiclip) and place the food near the bottom and both the fish and the bottom feeders will eat from it.
Last thing I think is very important. Like food for us, quality matters. I personally recommend Hakari based products, as they seem to be of very high quality, and if you compare their stats (all bags will have minimum %'s for protein, fat, etc) they seem to be superior. The shrimp pellets I got were by TopFin, and I wonder if they are less desirable to the fish because of some poor ingredient added, moreso than the actual product.
Bottom line, just like us, your fish want and NEED a varied diet. A good mixed suppliment of high protein, low fat, with vegetable matter will keep them all thriving and showing their best colors.
HTH
justin