The quick answer is no, there is no need to do regular water changes in garden ponds, however there are occasions when you might need to do a water change due to water quality.
The most common reason for doing a water change is to dilute the amount of pollutants in the pond water.
Evaporation removes a fair bit of water from your pond which is usually replaced by rain water, this is natures way of doing a water change, but unfortunately the evaporated water doesn’t take away the pollutants, your filter system is left to remove the pollutants and usually does a great job at doing so, however there can be a point when the pollutants get too much and the filters just can’t cope any more, this is where a water change should be considered.
The amount of fish you have in your pond will play a factor into how much pollutants you have in your pond as they produce waste (poo), the size of your pond can also be a factor, the smaller the pond the easier it is to get polluted, a large pond has more water to help dilute the pollutants.
If you have, or want to grow big fish in your pond, or your pond is small and overstocked with fish you will probably need to do some sort of water change at some point, but if you keep a reasonable amount of fish in a large pond the need for water changes drops dramatically.
During the winter months when your fish are less active and the water parameters are more stable you will not need to do water changes.
Our preferred method is the drip system, this is where you constantly add fresh water to your pond, you will need an outlet on the opposite side of your pond otherwise your pond will over flow, a properly installed overflow would be the best option.
By using the drip method you take your pond from a closed system where the body of water is sustained within your pond to an open system were the water is constantly changing, think of it being like a lake being feed by rivers (inlet) and then the lake feeding other rivers (outlet).
The easiest way to build a drip system is to feed your pond using an outside tap and a garden hose, you can add an inline dechlorinator to the hose to make the tap water safe and free from chlorine, but the amount of water you’re adding is minimal and should dissipate as fast as it’s added, then slowly turn on the tap to get the right amount of water to run into your pond, it’s not a drip you’re looking for unless you have a really small pond, it’s more like a trickle of water, the bigger the pond the bigger the trickle.
Fish like this method because they get a constant supply of fresh water, the pond is always topped up so you dont have to worry about evaporation and it’s a set it and forget it system.
Partial Water Change.
Another option is doing a partial water change of about 5% – 10% of the pond water once per week until the water quality returns, you can remove the water with a bucket or by syphoning but check you’re not scooping out any fish or wildlife.
When adding fresh water from a tap it will need to be treated, tap water contains chlorine or chloramines which are toxic for your fish.
Complete Water Change.
We would never recommend a full water change to correct a problem with your water quality, a total water change would cause a massive shock to everything in your pond, your fish, plants, beneficial bacteria and algae would suffer massively.
Ok your pond would look good for a couple of weeks, but your pond would be sterile with hardly any bacteria in it, it would be full of chlorine and toxic heavy metals, it would quickly turn green and your fish will suffer.
If you need to remove all the water from your pond, maybe to change the pond liner due to a leak, try to store and reuse the same water rather than using fresh tap water, if you have to use use tap water to fill the pond again you will need treat the water with a dechlorinator and add some benaficial bacteria into the pond, this will help your pond settle in and will be a lot safer for your fish.