I know of some people that have a separate smaller tank in order to breed guppies for this purpose. Although I don't have any experience with guppies myself, I've heard that thay are pretty prolific. I'm actually thinking about setting up a nice little tank in my office just for this reason. It's an excuse to get another tank and will hopefully supply a decent batch of fry from time to time. Whether you go with convicts, guppies, or any other fish as feeders it's best that you raise them in a tank of your own where you can be assured of water quality and the general health of your supply.
I currently feed my residents an occasional treat of either live brine shrimp, or live earthworms. I limit the live food to once a week(at most) so they will not get spoiled and refuse to take flake, pellets or any of the other dried products I have. Also, be sure to research the diets of the fish that you are keeping. Certain live foods can be ultimately unhealthy with some species that you may be keeping. Since most fish will eat anything that is put in front of them, there is a real danger of some of the residents eating too much of things that they shouldn't. Certain species of cichlids, for example, have primarily vegetarian diets in the wild and will develope bloat or liver disease if they are fed too much protein or fatty foods.
