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Old 02-26-2020, 04:39 PM   #1
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Fasting Fish

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
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Old 02-26-2020, 05:17 PM   #2
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I've always wondered about the plus/minus effects of fasting. I *sort of* understood the reasons given for doing it but had never searched for proof to back up the reasons given. I have only ever fasted fish before an auction to keep the bagged water as *clean* as possible so seeing that it isn't a recommended practice I'll stick with alternating regular and *smaller* feedings as opposed to *no* feedings

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Old 02-27-2020, 12:25 AM   #3
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I think this doesn’t quite take into account the chronic overfeeding of some aquarium fish and that some fish are indeed prone to swim bladder issues from constipation.

In a lightly/ moderately stocked tank a fast day gives the fish incentive to forage; eat leftover food that went unnoticed or the plethora of micro crustacean life that thrives in a well established aquarium. And it serves as a protective measure against the constipation and swim bladder issues in some species of fish that are prone to it.

You are correct that it is not a universal good but I would argue that you can’t say it’s a universal negative either. Some tanks / species will do better with a fast day, some it would be neutral or harmful (heavily stocked tanks and not chronically over fed fish for example.) As with most things you need some context to decide if a fast day occasionally would be a benefit to your particular community.

My glofish tank does not; it’s heavily stocked and they’re aggressive eaters. My shrimp tanks are full of foragers and Pygmy cories who eat the micro life in the tank. They only get fed a few days a week.
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Old 02-27-2020, 01:39 PM   #4
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Hello rl...

If you feed a small variety daily, the fish will be fine. Most have a stomach the size of one of their eyes, so they don't require much food. A fasting day is fine, but if you change half or more of the tank water weekly, you'll remove any waste material that's dissolved in the water. If you keep the water keeping hobby simple, you'll be more successful.

"Fear not, my young apprentice. Just change the tank water."
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fish, sting

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