Please do not feed them human cereal. The foods mentioned above, prepared for different kinds of fish and their needs, are what you want. You may also include cucumber slices, zucchini, lettuce, but remove uneaten food after an hour or two as it will contribute to bioload along with fish poop. Are you doing partial water changes and gravel vacuuming?
yes, prepared fish foods already contain carbohydrates. especially if you use pellets or flakes, you will see that they add such things as wheat flour, soy, corn meal, even potato.