How to restart your koi or goldfish pond in Spring – after a long cold winter

Whether you like it or not, at this time of the year all ponds have two things in common. They will have experienced a winter period (of varying descriptions), and will be fast approaching the immovable deadline of spring by when any jobs need to have been done in readiness for your pond’s return to becoming fully operational.

Even during the months of relative inactivity over winter, your pond and it’s life-supporting hardware will have undergone significant changes having experienced the physical extremes of our climate. Now is the time to take stock and respond to your assessment of the pond and it’s equipment, ensuring that winter merges seamlessly into spring and then on into summer.

What state will your pond be in at the moment?

As the majority of koi keepers do not heat their pond over winter, for this article I will assume that the pond has not been heated but followed ambient temperature over winter. During this period your cold-blooded koi will have become very inactive (through no choice of their own), with their metabolism barely ticking over, utilising stored reserves to meet what little energy requirements they may have. As there has been no food added to the pond, there will have been little incentive to carry out a water change through the cold months. So it is highly likely that the composition and analysis of your pond water will have changed, and with the changes that are going to take place as spring approaches, the pond will experience another dynamic period. Our role in managing our pond and koi at this stage is to ensure that the changes are as smooth and as natural as possible, bearing in mind that sudden and extreme changes in the pond environment are likely to be intolerable for our koi, leading to their stress and the related health problems.

What are we to expect as spring approaches?

Unfortunately, spring approaches in the same haphazard way that last winter approached, with the gradual seasonable change in weather being punctuated with the odd days of freak unseasonable weather. Our koi’s responses will not be as extreme as these daily changes on account of the pond water’s ability to dampen the isolated fluctuations in air temperature. Nevertheless, if we recorded our pond’s water temperature as spring approached, we would plot a gradual rise in water temperature, and notice an increase in koi activity. There are three phases to a pond’s recovery as the water temperature increases.

4C. Above this temperature, biofilter activity will move from being inactive or dormant to becoming functional again. Bacteria will start to divide (still as slowly as they would do in a fridge) and commence consuming the array of inorganic and organic material that would have accumulated over the winter.

6C. At around this temperature your fish will start to become noticeably more active, swimming slowly, but with intent rather than the aimless drifting that you may have witnessed at lower temperatures. Your fish will still not be interested in (nor require) feeding – (but there’s always the odd one that breaks the rule and hasn’t read the textbooks – usually a Chagoi!)

10C. Your koi will have been showing regular interest in food, with all koi feeding at this temperature (but still at reduced levels compared to summer temperatures). Naturally, for your pond’s benefit, and you’re koi’s requirements, they should only be offered a low protein food.

What effects will the last year have had on the pond equipment?

You are weeks away from firing up your pond again for spring, which gives you a chance to carry out a situation check on your pond equipment. This will hopefully reduce any mishaps or equipment failures later on in spring when your pond and koi’s well-being will depend upon it.

UVcs. Whether you have opted to run your UV through the winter or not, now is the time to check that the winter has been kind to the exposed plastic. If exposed to direct sunlight and then frost, the combination of these extreme elements may well have led to plastic work (including host tails) becoming brittle.

Two key areas of concern will be your quartz sleeve and UV bulb. If you have put your UV unit in dry dock over winter, the quartz sleeve may have become fouled and encrusted with a combination of organic mulm and limescale. With care, and while checking the integrity of your O-rings, remove the quartz sleeve and if necessary clean the outside, especially if there is any sign of a tidemark. But beware as the quartz sleeve is very prone to cracking if not handled with care.

This is always the best time to replace the UV bulb as it will be facing it’s biggest challenge in the preceding months. Even if your existing bulb still lights up, it’s UV output will have reduced significantly over the past season. If you’re not sure which bulb you will require, take your old one to your koi dealer to match it up with a new one.

Pump. Ideally, you would have removed your pump from the pond when you turned it off at the onset of winter. You can then dry it off and Vaseline any frictional surfaces in readiness for the switch-on in spring. However, if pipework has made difficult to remove, now’s the time to bring the pump to the surface for a service. Remove any external straining device or basket and soak in hot water containing bleach to remove any stubborn algae or tufts of blanket weed. The bleach will kill all this, preventing it from growing back at a pace, rapidly restricting the water intake. Also, depending on the pump’s construction (and serviceability), if possible, carefully remove the impeller and it’s housing to check movement is still free and unhindered by any accumulation of silt over winter. I have found that smearing any exposed metal services on the impeller or pump body ensures a smooth start when the pump is started again in spring.

Pipework and filtration. Filters and pipework will have inevitably gathered particulate debris both before and during your winter shutdown. One potentially catastrophic event that you must plan to avoid is for your pump to blast new water through your rested filter and pipework, taking with it a highly anoxic and potentially toxic organic sediment that will have settled in that pipework and filter. Even at the biologically inhospitable winter temperatures, bacteria will have exhausted the oxygen in and around the settled layers of sediment, turning them anoxic. The anaerobic bacteria will have taken up residence in this silt, converting organic and inorganic products into toxic by-products. If these are released into your pond not only will your koi experience an unnaturally rapid deterioration in water quality (most of us can remember that tell-tale bad egg smell) but the water quality will almost be irretrievable, having to look at ditching the majority of your tainted pond’s volume to retrieve the situation.

Anoxic layers can also settle in the pond itself, so take care when pulling up your pump from the pond bottom and look at vacuuming your pond bottom very gently as soon as possible. To avoid intoxicating your pond with settled debris, divert the first few hundred gallons of water after your initial switch-on on to your lawn. If you have discharge valves in your filter chambers pull those to waste also. If you have a waterfall, you will find it impractical to divert it’s first flow away from the pond, so you will have to remove the leaf matter and silt manually from the channel before switching on the pump.

Air pumps and diffusers. These will not have been used over winter as DO at low temperatures is at it’s highest, while a pond’s demand for oxygen is at it’s lowest. Also, vigorous water movement made by air diffusers mixes upper colder layers with the protective warmer lower layers (something which must be avoided in winter). If you’re fortunate enough to have a bottom drain-fed filter, with your bottom drains converted to air domes then you may encounter a few teething problems upon re-inflating them this spring. Bacterial and algal growth will have plugged some of the perforations, while the membrane is not at it’s most flexible at these low temperatures, making it less likely to blow off any stubborn debris. In extreme cases, you may have to consider entering the pond and removing it for a hands-on service, something that may involve dropping the pond down by a few feet.

A regular perishable item in air pumps is the diaphragm. These can fail unpredictably, but more so when the diaphragm is cold or has not been used for several months. Keep a spare one handy just in case, but also keep an ear open for the tell-tale increase in noise from your pond’s body that will indicate a diaphragm is about to fail.

Pond cover.

Whether you have merely covered your pond with a net to keep predators and leaves at bay or have gone a stage further by constructing an insulated pond cover, now’s the time to remove it and assess any damage. The degree of damage will be determined by how hard a winter we’ve had (and how easy the framework is to dismantle!)

Water Tests

Once you have addressed any pond hardware issues, you can take stock of the condition of your pond by carrying out a comprehensive water analysis. If you’ve got the range of test kits (that are still in date) then you can test away, or if not, take a water sample to your koi dealer who will test the key parameters for you and even suggest any remedial action in the light of the results.

pH. It is likely that pH will have drifted downwards, becoming more acidic as winter rain, sleet and snow would have diluted any natural buffering. A dropping pH will soon be adjusted gradually once you recommence the circulation through your filter containing a calcium carbonate substrate. As part of your spring clean, make sure that the surfaces of your buffering media are cleaned of any winter sludge and slime.

Ammonia and nitrite tests will not be necessary, as your fish will not have started feeding yet. But why not test them once out of curiosity just to check that they are zero. If either test reads positive, check your test kit.

Oxygen will also not require testing as it will be abundant at these temperatures and the demand for it from within the pond will also been negligible.

Nitrate. This could prove to be an interesting test, and be prepared to see a result showing either extreme. It could be high due to the low-level processing of ammonia (and nitrite) released in minute levels over winter or it could be low due to the diluting effects of excessive rain water. Only intervene if levels are above (or approaching) 50 ppm.

KH and GH. These will also back-up your pH reading and indicate your pond’s current buffering position. These should be maintained at good levels through a combination of regular water changes and a buffering filter media.

Pre-season pond treatments. If you routinely adopt a ‘preventative’ strategy of dosing your pond in spring with anti-bacterial and anti-parasite treatments before your fish become active, be sure that any formalin based treatments have been stored suitably well over winter and that all other treatments are within their use-by dates.


Your single greatest concern (and critical success factor for seamlessly introducing your pond and fish back into spring activity) will be managing your filtration correctly at this preparatory stage of the season. For those koi keepers who have maintained a flow (however small) of water through their filters, the risks will be small once the fish start feeding and releasing ammonia in increasing amounts. These filters will have retained the maturity that has built up and benefited your pond over the previous season. The density of the bacterial population will have declined (due to the drop in available food and water temperature) but you will have retained a diverse population of bacteria that will include all the autotrophic and heterotrophic bacteria required. That is, the bio-filter will contain members of the widest range of bacteria that will immediately process a diverse range of inorganic and organic wastes as water temperatures increase in spring. You will not have to wait for different bacteria to colonise naturally (and slowly) as you would in a new pond, often flying very close to avoid New Filter Syndrome.

If you have not kept your filter running over winter, you will be at a great disadvantage come springtime, when you and your fish start to demand a higher degree of filter performance once feeding starts. A filter that has been allowed to dry out over winter will have lost it’s diverse bacterial population as well as the associated comprehensive waste processing capabilities. This could well lead to ‘New Pond’ type water quality problems where the rate at which waste is produced by your fish (and in some instances other bacteria) exceeds the capabilities of the biofilter to break it down, leading to an accumulation of toxins in the pond water with the associated fish health problems. A filter’s performance is constrained by the rate at which different and successive populations of bacteria are able to colonise a new filter until between them, the whole team of bacteria exploiting their own and different niches will cope with the diversity of wastes that a pond produces.

If your filter has not been running over winter, then take this opportunity to give all media (mechanical and biological) a good clean. If you have a friendly and understanding fellow koi keeper (or koi club member) who may have heated their pond over winter and retained a diverse and thriving bacterial population, they may be persuaded to loan you some mature biomedia that can be temporarily housed in your filter to seed new life into your immature media.

What’s the deadline?

Speaking personally, I find that nothing focuses the mind better than an immovable deadline. In your pond’s case, you should aim to get your spring clean complete, and your rejuvenated filter running when pond water temperatures have not yet reached 8oC. Bear in mind that this is the temperature at which fish will consistently start to feed, but also be aware of the risk of isolated hard frosts that could chill your circulating pond overnight.

In summary, every koi pond is different, with variations in hardware, stocking levels and local climatic conditions affecting the actions required to ensure a healthy transition from winter to spring. However, there are a few critical success factors that are common to all koi ponds, which are all quite logical and easy to carry out. Where problems will arise is in areas that we can’t fully control (such as the date of arrival of spring and un predictable weather in between) but with careful and flexible planning an incident-free transition from winter into spring should be well within our grasp.

Scheduled Tasks

De-silt pond bottom, pipework, filter chambers

Clean and service pump and strainer.

Clean out filter chambers, being careful to dump any stagnant water

Service UV and replace/clean parts as required

Test water and respond accordingly. Continue to do so upon first feeding.

Run in filter and seed if possible with mature media

Air pump, diffusers and airline. Replace as required ready for later on in the year when aeration will be required.

Cleaning Tools

Quartz Sleeve: Low-abrasive pan scourer (for non-stick pans) and hot water Pump Strainer: Bleach and hot water, stiff scrubbing brush Airdome membrane: Hot water, scrubbing brush (if you’re able to retrieve it from the pond bottom!) Mechanical Media (dry and encrusted). Pre-soak and then blast with a high pressure hose.

Replacing Equipment

Jubilee clips Grease every year UV Bulb(s) 1 Season Airline 1-2 years O-Rings (UVc) 2 years Diaphragms 2-3years Foam Media 2-3 years Pump 3-5 years Quartz Sleeve 3-5 years (depending on limescale) Flexible hose 3-5 years (depending on exposure to UV etc)

Kill blanketweed and string algae.