I find the idea of fasting fish a sort of an aberration. Granted, it would result in less waste but then fish don't have much energy reserves since their bodies store little fat. A fish is a small animal which means it has a large surface area to volume ratio. Such animals need constant feeding to replenish their energy requirements as opposed to bigger animals with a bigger volume to surface ratio which is more effective in conservation of energy in the body. In any case, I think this would result in hypotonic conditions in its blood. I posit that fish are delicate creatures and something which is out of the ordinary should be seldom tried. The nitrogeneous waste which they excrete into the water is also important in the ecosystem because it enables the survival and growth of water plants like Elodea. Fish are also filter feeders and do feed on dead organic matter in the water called detritus which is more or less always present. This means that the idea of fasting fish for a few days especially when you are absent is not true to fact. At the end of the day, nature has a way of correcting itself through positive and negative feedback mechanisms.
