I know this doesn't apply to a pond setting, i.e. it's hard to know the answer with fish in a pond where they they have natural food to eat, but is there a way to determine how much of the food the fish is actually utilizing and how much of the food (when I say food I mean nutrients in the food) is passing through the fish and is not digestible?
I'd like to offer some subtle improvements to this question. The conversion of anything a fish eats, whether it eats a formulated feed or natural prey item, is less than perfectly efficient. It requires energy to assimilate food and so some of the food is digested just to provide the energy. The waste product of this conversion to energy is not food that is not digestible. Instead the wastes are products of oxidation which is part of the digestion process. For protein, the products are primarily ammonia and carbon dioxide both of which are expelled as water soluble wastes. The better question is how much of the feed is assimilated because everything the fish doesn't retain leaves the fish.
