It depends on how big the wafers are and how big the cories are. I had wafers that were the size of a (flattened) chocolate chip(Hikari sinking wafers), and wafers that were size of two nickels stacked on top of each other(Aqueon algae rounds). For my three albino cories I feed 1/4 of the nickle-sized pieces and 1/2 to 3/4 of the flat chocolate chip sized ones. But I feed at night otherwise the betta would eat the WHOLE wafer (plus some fins), not just part of it. Anyways, you really have to imagine how much they can eat and think of how much of the wafer that would be. A whole one is generally too much unless you have other fish in there.