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Comments about digestibility of food and the quantity of waste produced are something that always bother me because I am not aware of any study showing greater absorption of one food over another. Very little of the dry content of any food is incorporated into the living tissue of an animal. I think these comments derive from observation of the physical properties of the waste produced. Some foods will result in gelatinized fecal matter in long visible strands. Other foods will produce small particulates which do not settle out of the water column as readily. Koi feeding on algae exclusively for several days (such as before a show) will produce small pellets of waste having an appearance similar to rabbit/guinea pig waste ... and dark green in color like the algae. In a settlement chamber these wastes will settle at different rates and will give a different appearance to the system waste discharged from the settlement chamber.
It seems to me that, as with mammals I'm aware have been studied, the food which moves a bit slower through the digestive tract likely allows greater nutrient absorption, but results in the long highly visible strands of waste. On the other hand, the food that moves through the system rapidly and has a laxative effect likely has less nutrient absorbed, but the waste is of such fine particulates that it will seem that there is less of it because less is observed in the settlement chamber, but there is more of it suspended in the water column to be entrapped elsewhere in the system or to settle out on pond surfaces for decomposition.
I suppose I'm feeling contrarian today.
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