There is lots of anecdotal information on fasting fish - not feeding them once a week or every other day or on weekends. The reasons given are cleaning out digestive tract or letting the fish clean up scraps. But no evidence is cited,just stories, so I perused the scientific literature and : according to several experiments, fasting fish have their digestive hormone balance disrupted for the fasting day as they have been acclimated to a certain pattern the other days of the week, and their glucose level drops. Second, fish feeding in the same area, especially those of the same species, develop a social/dominance order during feeding time and not feeding them temporarily disrupts this order.
That said, fasting is not particularly harmful to fish and they recover quickly. So it's ok to leave them for a day or two or week without food.
But fasting does not help the fish so although it is not harmful, it is not beneficial.
Roger (always pH) Lederer