Overfeeding is a mistake many hobbyist make, at least, in the initial stages. Overfeeding means that the fish do not eat all the food. The uneaten food decomposes and contaminates the water. Overfeeding also causes digestive problems to the fish and dirties the water in the aquarium.
A general rule of thumb is to ensure that the food you drop into the tank is consumed within 5 minutes. If the food remains uneaten for more than five minutes reduce the amount during the next feed and you will soon know the optimum amount to be fed.
The amount of food will vary with the time of day and with the number of fish.