How to drain a garden pond without stressing the fish

Draining a pond completely is rarely necessary, and often counterproductive. But after a pollution incident, a liner repair, or when the sludge has built up to several tens of centimetres, it is sometimes the only solution. Here is how to do it without putting your fish at risk.

Do you really need to drain?

A balanced pond does not need draining. A thin layer of sludge and the biofilm on the walls are beneficial. Before emptying everything, consider:

• A pond vacuum to remove sludge without draining.
• Sludge-reducing bacteria.
• Partial water changes of 10 to 20 %.

When to drain

In early spring (March-April) or early autumn (September), when the water is cool but not cold. Avoid summer, when fish are more easily stressed and oxygen runs short in holding tanks.

Step 1: prepare the holding tank

Fill a large tank or tub with water from the pond itself, not tap water. Place it in the shade, add an air pump and cover it with a net to stop the fish jumping out.

Step 2: pump and catch

Pump until only 20 to 30 cm of water is left: the fish are then easy to catch with a landing net, without chasing them all around the pond.

Step 3: clean without detergent

• Remove the sludge with a shovel or a pond vacuum. It makes an excellent fertiliser for the garden.
• Rinse the walls with clean water, no soap, no bleach, no powerful pressure washer on a liner.
• Take the opportunity to check the liner, divide plants and clean the pump.

Step 4: refill

Refill and add a water conditioner that neutralises the chlorine and metals in tap water. Keep the filtration running.

Step 5: reseed

Draining removes a large share of the beneficial bacteria. Add nitrifying bacteria to the filter as soon as the pond is refilled.

Step 6: reintroduce the fish

Place the fish in a floating bucket filled with water from the holding tank, and gradually add pond water over 30 to 60 minutes to equalise the temperature. A difference of more than 2 to 3 °C is a shock. Do not feed for the first two days.

Read also

For a successful biological restart, read Pond bacteria: the complete guide.

READ THE ARTICLE

What to prepare the day before

A smooth drain-down is decided before the pump is even switched on. Allow a full day, mild and overcast weather, and a helping hand: two people go much faster. Gather the equipment: tank with a lid or net, aerator, landing net, buckets, hoses, pump, thermometer and water conditioner.

Stop feeding the fish the day before and on the day itself: they will foul the tank less and cope better with the stress. Also plan where the water will go. Direct it to the garden or a soakaway area, well away from the pond hole, without flooding your neighbour. If the pond is home to newts, dragonfly larvae or other creatures, keep some of the water and sludge in a bucket and put it back afterwards: this will help pond life return more quickly.

Keeping an eye on the fish in the tank

A holding tank is a very small environment. The temperature changes quickly and oxygen runs out. Switch the aerator on before putting the fish in, and check the temperature every hour. If the water warms up, add shade rather than tap water. Catch the fish with the landing net, then support them with a bag or bowl to avoid damaging their slime coat and fins. Never leave large fish without a lid: they can jump very high.

Watch their behaviour. Fish that stay at the surface gasping for air are signalling a lack of oxygen: increase aeration and shorten the holding time if possible. Also avoid feeding them during this period, and do not put more fish in the tank than necessary.

The following weeks

A pond filled with fresh water restarts almost like a new pond. Test nitrites a few days after refilling, then once or twice a week during the first month: a spike is always possible until the filter has regained its balance. Our article Nitrites in the pond: emergency measures tells you what to do if the reading rises.

Resume feeding in small amounts, keeping an eye on appetite. Also check the KH, which can change with the top-up water. If you have rinsed the filter, do it with pond water, as explained in Cleaning your pond filter without killing the bacteria.

Frequently asked questions

How long can the fish stay in the holding tank?

A few hours to a day, with aeration and in the shade. Beyond that, set up a small filtration system.

The water turns green after draining, is that normal?

Yes, frequently. The new water is rich in nutrients and the bacteria are not yet established. It stabilises within a few weeks.

Should you drain every year?

No. In a well-filtered, well-maintained pond, draining once every few years, or even never, is enough.

In summary

• Only drain if necessary, in spring or early autumn.
• Keep the fish in aerated pond water, in the shade.
• Clean without products, and keep the sludge for the garden.
• Conditioner, bacteria, then a gradual return of the fish.

Last updated: 27 September 2026.