Many aquarium hobbyists propose fasting their fish once a week (i.e. not feeding the fish one day every week). The benefits of doing so include:
- lower chance of constipation / bloating which may lead to swim bladder disease
- clear the digestive system
- better water quality (this is not the main point, but a side benefit)
I think in part people fast fish because in nature, if the fish was living it’s natural life, it would never be able to count on finding food every day, so fasting now and then is seen to be more in keeping with a natural life. Living in a glass box is so far from nature I’m not sure how valid that reasoning is, but it does no harm.
Man made foods are also filled with things the fish would never, ever eat in the wild, so fasting may be a break from some of that. Might be better to alternate man made with frozen or live though, the fish will be healthier for it.
If a fish has been constipated, fasting may well give the bowel a needed rest. Man made foods have many things like flour and other binding or bulking agents, and they are, I think, one of the prime reasons some fish become constipated. It’s not what they evolved to eat.
Just for an example, feeding living daphnia in quantity to fish is almost like giving them a laxative, they shoot through so fast, but fish eating living daphnia have amazing colour and vitality too. Great for conditioning to breed. In the wild daphnia are rather ephemeral, having huge numbers in spring and far fewer later. So fish and other critters that eat them, eat them much like we do some of the more perishable fruits.. when they’re available ! They eat all they can ’til they’re gone.
There is also a scientific research article on fasting tilapia fish. Basically the study found that “Overall, results suggest that fasting one day a week does not affect growth efficiency and that tilapia adjust well to relative changes in feeding frequency.”