From the link posted above .....
Feeding fish is part art, part science. As far as how much, the easiest method for the average fish keeper is let your eyes be your guide. If the fish is looking skinny, feed more, if the fish is looking obese, cut back & feed less. This will vary among species, even among fish within the same species, regardless of overall weight/size, due to the energy expended by the fish in a glass box. So in order to properly meet those energy requirements, and depending on how nutrient dense your food is, or isn't, can cause WIDE variations in what each fish requires for optimum or ideal growth. Commercial vendors clearly have different goals than someone raising a pet fish in a 90 gallon.
Do not feed to satiation as most fish are hard wired to overfeed. (feast/famine)