Pond water changes

As koi keepers we are experts in keeping fish in an enclosed artificial environment that is stocked with an unnaturally high stocking level and fed a relatively large quantity of food.

Consequently, there will come a time when the pond’s ecosystem becomes unbalanced, requiring our input and intervention (yet again). In the short term, a mature biofilter manages to keep on top of the day-to-day balancing act, by processing and detoxifying the steady supply of ammonia and other toxic products produced by fish on an hourly basis.

This allows our koi to enjoy a relatively stable and unpolluted environment. Yet over a longer time frame, (weeks and months), our pond water will naturally start to accumulate a multitude of other processed by-products that if our pond was natural, would simply be processed further and taken up by other living organisms in and around the pond.

However our unnatural pond suffers from truncated nutrient cycles, meaning that the various elements have been detoxified they cannot be totally recycled (as would happen in a natural, balanced environment) only to accumulate in our pond. These intermediary compounds are quite safe at specific concentrations in a pond (usually measured in parts per million) but over time, could accumulate to levels that would begin to adversely affect our koi directly, or indirectly by unbalancing the pond environment.

Another reason for carrying out a partial water change is to add minerals to a pond that may not have had the benefit of regular clay treatments. If you’re from a hard water area, then a regular water change will help to refresh the mineral content of your pond.

You can use a combination of information gained both visually and by your test kit to determine whether your pond would benefit from a partial water change. Over time, a pond will accumulate dissolved organic compounds from food and processed organic waste, giving the pond water a yellow/brown tinge.

When you see your pond every day, this gradual deterioration in water colour can go unnoticed, as the change can be so slow. Why not check if your white koi really do look white (and not creamy) at the bottom of the your pond.

A useful benchmark I use to determine whether a partial water change is due is to test for nitrate. This is the ‘bank’ where the nitrogen from the ammonia is ultimately deposited, increasing in concentration as more nitrite is oxidised into nitrate by bacteria in the biofilter. Levels greater than 50 ppm should be avoided, but set yourself a target of keeping them below 25 ppm.

Other cumulative compounds (sulphates, phosphates etc) will have a tendency to accumulate but rather than test these individually, use the nitrate test as a useful benchmark (i.e. if the nitrate levels are increasing then it is highly likely that the others will be also). You should check your nitrate levels every two weeks or so in the warmer months, with no need to do so in the months when you’re koi are not actively feeding.

How partial is a partial water change?

The objective of carrying out a partial water change is to maintain a stable, quality pond environment for your koi. This means that frequent small water changes are preferred over fewer, larger water changes. Using your nitrate reading as a guide, you should carry out a partial water change (approx 20 per cent) as frequently as is required to keep it below 25 ppm. This will vary between ponds (because of differences in feeding, stocking, filtration etc) but will tend to work out to be a fortunately operation.

The water that you use to replenish your old pond water should ideally be passed through a suitable water purifier. The water purifier will intercept and retain any undesirable contaminants in tap water that will also accumulate in your pond, causing stress to your fish. You may find it useful to test the nitrate levels of your purified water as it enters your pond to check that it is less than 25 ppm. If it is not, you will not be able to dilute the nitrates (as well as other compounds) in your pond using this source of water.

You may be restricted by the flow rate through a purifier as to how much water you can change within a convenient period. For example, a 20% water change for a 10,000 gallon pond (2000 gallons) will demand more throughput through a water purifier than a similar water change in a 5000 gallon pond (1000 gallons). If your purifier struggles to deliver sufficient water for your pond, you may have to look at buying a larger purifier, or carrying out small water changes more frequently.

Tap water conditioner?

The alternative to using tapwater that has been passed through a purifier is to add a quality tapwater conditioner to raw tapwater. Sufficient should be added to treat the volume of newly-added tapwater (rather than the whole pond). A quality tapwater conditioner will remove harmful chlorine and chloramines and bind up heavy metals and other potentially harmful contaminants. You will need to know the volume of your water change to dose accurately.

Another concern you should have when contemplating how much water to change is the effect that a sudden influx of cold water may have on your koi, especially in the height of summer. If, having dropped your pond level by 20%, you are unable to retain pump and filter circulation, then at least keep your pond aerated as this will help mix new and old pond water. If your filter chambers are wet (rather than a pump-fed trickle filter) then keep them aerated also using air stones.

Koi generally show a positive response to a water change, with very obvious changes in behaviour, typified by increased activity, vigour and appetite. So much so, that you should be sure to keep a watchful eye on your koi during and shortly after a partial water change as they can often jump clear of the surface during this period.

Once you have carried out your partial water change re-test the nitrate levels to measure what positive effects the water changes have had on your pond. The results will help you plan when you’re most likely to have to carry out your next partial water change.

Top tips for a successful water change: step-by-step guide.

Test and record your nitrate level to confirm your pond requires a partial water change.

Remove 10-20% of your pond water first (by vacuuming or from discharging your primary filter chamber or bottom drain).

If pump/water flow stops, ensure your pond and chambers of aerated sufficiently for the duration of the water change.

Add water that has passed through a purifier, testing that the nitrate levels in the new water are sufficiently low to effectively dilute the nitrate levels in your pond. If you don’t have a purifier, add sufficient tapwater conditioner.

Keep a check on your koi both during and after a water change as they can often jump clear of the water upon experiencing some fresh/new water.

What you will need.

Nitrate test kit, to check before and after.

Water purifier (ideally) – otherwise, a good quality tapwater conditioner

Hose, dedicated for your pond use.

Discharge facility to waste (drain from filters, bottom drain or discharge directly from your submersible pump)

Kill blanketweed and string algae.