While it might seem simple, improperly feeding your fish can lead to big consequences. Not to mention, due to profit motives, fish food will generally tell you to feed more than is necessary so that you run out of food quicker. In reality, fish only need to be fed once daily. The amount of food is dependent on how many and of what kind of fish you have. In the absence of bottom feeders, you should only feed as much as the fish can eat. You shouldn't be seeing much fall to the substrate at all. If there are bottom feeders, over feeding slightly is the best practice to ensure proper nutrition for them. Just feed a bit more than all of the mid to upper swimmers can catch, so that some falls to the bottom. Your fish might still be begging for food when you finish feeding, but as a general rule of thumb, their stomachs are about as big as one of their eyes. That's it. That's all the food they really need in a day. A lot of fish that need extra food will spend a large majority of their time scavenging the substrate, decor and other surfaces for food to eat, including algae. Mollies are a good example of this. I can feed my tank heavy, and still see them picking at everything soon after. It's a natural behavior, but it does not mean your fish are starving.