Can you continue feeding your fish while treating the pond?

The bottle is emptied, the pond is treated, and at 6 p.m. the koi gather at the edge as they do every evening. The question then arises naturally: should I feed them or not? We are asked this at the Aquiflor counter almost every week in spring. The answer depends less on the fish than on the product used: a medicinal or oxidizing treatment calls for a short fast or a greatly reduced ration, whereas bacteria or minerals generally require no major change. Here's how to decide, case by case.

Why Food Matters So Much During Treatment

Everything you distribute eventually ends up in the water in one way or another. Some of it is assimilated by the fish; the rest comes out as excrement and ammonia excreted through the gills, while uneaten pellets decompose on the bottom. In a balanced pond, the biological filter absorbs this load without difficulty. During treatment, this is no longer the case for three reasons.

First, many products used against parasites or pathogenic bacteria also temporarily inhibit the filter's beneficial bacteria. The ability to convert ammonia into nitrite and then nitrate declines at the very moment when ammonia production continues. Next, some treatments consume oxygen or work through oxidation: digestion also increases the fish's oxygen needs. Finally, a sick fish eats poorly. Pellets that float and then sink are lost to the fish but add to the pollution.

Boyd and Tucker point this out in their reference work on water quality in pond aquaculture: feed is the primary source of nitrogen in a stocked pond. Reducing the ration therefore directly affects ammonia and nitrite, without using any product.

What to do depending on the type of treatment

This table provides guidance, not a universal rule. Some manufacturers explicitly require stopping food completely for the entire duration of the treatment, while others say nothing about it. When the instructions specify something about feeding, they take precedence, even if your usual practice is different.

Temperature changes everything

A fish is a cold-blooded animal: its digestion slows as the water cools. In spring, when antiparasitic treatments are most frequent, the water often fluctuates between 10 and 15 °C. At these temperatures, a koi digests slowly, and a heavy meal remains in the intestine for a long time. Add a treatment, and you have a fish that is already slowed down and is being asked to digest and defend itself at the same time.

In practice, at around 12 °C or below, we recommend giving only a wheat-germ-type food, in a very small quantity, and only if the fish really demand it. Between 15 and 20 °C, halving the ration for the duration of the treatment is a good rule of thumb. Above 20 °C, dissolved oxygen becomes the limiting factor: warm water contains less of it, and that is when a treatment combined with heavy meals can push a pond over the edge toward the end of the night.

The signs that say “stop feeding”

Even outside the manufacturer's instructions, certain signs mean you should stop feeding for 24 to 48 hours:

• fish that “gulp” air at the surface early in the morning;
• pellets still visible five minutes after feeding;
• an unusual smell or persistent foam on the surface;
• an ammonia or nitrite test that rises above zero.

A fast of a few days poses no problem for pond fish in good condition. They have reserves and, outdoors, always nibble on a little algae and larvae. The risk of starving your koi during a week of treatment is infinitely lower than the risk of poisoning them with their own waste.

How to resume feeding after treatment

Resuming feeding takes place in three stages. On the first day after treatment ends, give a small pinch, which the fish should finish in two or three minutes. If everything is eaten and the tests remain clear the next day, increase the amount gradually over four to seven days. Return to the normal ration only once the biological filter has restarted, which can be checked simply: nitrite remains at zero despite feeding.

This is often the right time to reseed the filter with bacteria, especially after an antiparasitic treatment, and to support the fish's mucous membrane, which has been put under considerable strain. We regularly see ponds where the disease has been defeated but the weakened filter causes a nitrite spike a week later, precisely because feeding was suddenly resumed.

A common case

Common case: koi are treated for skin parasites in spring, the water is around 16 °C, and feeding continues as usual, twice a day. Three or four days later, a few fish remain at the surface near the waterfall. The nitrite test is strongly colored. Nothing mysterious: the treatment has slowed the filter, while feeding continues to add nitrogen. Fast for two days, increase aeration, perform a partial water change, then resume gradually: everything generally returns to normal without another product.

Read also

Before choosing a product: How to effectively treat your fish in case of illness?.

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Frequently asked questions

— How many days can a pond fish go without eating?

A healthy adult fish can easily go several days, or even several weeks in cool water, without eating. A 24- to 72-hour fast during treatment is safe.

— Should feeding be stopped when adding bacteria?

No. Bacteria do not require fasting. Simply avoid overfeeding, and switch off the UV if the instructions require it.

— What food should be given just after treatment?

A highly digestible food, such as wheat germ, in small quantities, before gradually returning to the usual food.

— The fish are begging; are they suffering from hunger?

No: koi and goldfish beg out of habit as soon as they see you. This behavior does not indicate a genuine need.

In summary

• Medication or oxidizing treatment: fast on the first day, then a reduced, easily digestible ration.
• Bacteria, minerals, clay: normal food, adapted to the temperature.
• The manufacturer's instructions always take precedence over habits.
• Nitrite, oxygen and uneaten pellets are the three signals to monitor.
• Resumption takes place over four to seven days, with the filter restarted.

Last updated: September 29, 2026.