What size filter do I need for my garden pond

There are many things you need consider when trying to work out the size of filter you need for your garden pond, in this article we will guide you through what you need to know and how to get the right filter.

Why do you need a pond filter

Your pond filter and pump are at the heart of your garden pond, their role is to keep your pond healthy and looking good.

The ecological balance of your pond is maintained by your filter and pump, adding a skimmer to your pond to collect all the larger debris and leaves and a uvc to kill algae is the perfect combination to keep your pond clean and ensure your fish and plants will thrive, in this article we will concentrate on the main filter.

Choosing the right size pond filter

When calculating the size of filter for your pond you need to take into account a few things such as, the volume of water in your pond, how many fish are in your pond, what type of fish you want to keep, the location of your pond and to a degree the shape and depth of your pond.

How much water is in your pond

First you need to find the volume of water in your pond.

Measure the width, length and depth of your pond, If you have a rectangular or square pond you simply multiply it’s length x width x depth x 1000, this will give you the volume in litres.

If you have different depths or an irregular shape to your pond may you need to take several measurements to get an average of the length, width and depth and then do the calculation.

How many fish

The amount of fish you have in your pond is crucial to working out what size of filter you need, and it’s not how many fish you have but the size of fish you have.

You need to look at the total length of all your fish per litre of water, you measure your fish from its nose to just before their tail (you don’t include their tail in your calculations) we would suggest 25mm of total fish length per 50 litres of water for the average pond keeper.

The more fish per 50 litres of water the bigger the filtration needs to be, lets say you have a 5,000 litre pond, if you took the 25mm of fish per 50 litres we suggest you would be able to have 2500mm worth of fish, but let’s say the total length of fish is 3125mm, this is 25% more fish which would need 25% more filtration.

The type of fish you keep also has an effect on filtration, koi produce twice the amount of waste (poo) than that of goldfish so will need twice the filtration.

So what size filter do you need

Let’s say you’ve worked out the volume of water in your pond is 5,000 litres, you would think that you could just buy a filter rated at 5,000 litres, but often that’s for unstocked ponds, as soon as you add fish that capacity drops.

Let’s assume the filter is rated for an unstocked pond, if you fill your pond with goldfish using the 25mm of fish per 50 litres of water rule we would suggest a filter that doubles the capacity of the filter to 10,000 litres.

If you want to keep koi you would suggest you quadruple the size of filter to 20,000 litres, this is if you use the 25mm of fish per 50 litres rule.

If the total of fish in your pond is greater than the 25mm rule you will need to increase the capacity of the filter, for example if you had 31.25mm of fish per 50 litres of water, that would increase the amount of fish by 25%, now you would need a 12,500 filter for goldfish and a 25,000 filter for koi.

We would like to point out that each filter manufacturer has its own way of calculating the capacity of it’s filter, some will assume you are having fish and will give you the capacity of the filter for both goldfish and koi which can make the calculations a bit easier.

The one thing we would say is that there is no such thing as over filtration of your pond, we would always advise to over filter your pond to future proof it, remember, fish will grow and reproduce adding to the total length of fish in your pond as years go by.

Other things you may need to take into account is how often you feed your fish, if you feed them more than once a day this could add an extra load on the filter of about 10%, sunlight is also a factor, too much sunlight can add an extra 5% and pond depth, if shallow adds a small amount to the filters capacity.