Frequently asked questions on ponds

What is the volume of my pond?

You need to know the volume of your pond for all sorts of reasons from knowing how much tap safe or treatment to add, to selecting the right pump, filter and UV.

Measure your pond’s maximum length, width and depth in metres then multiply them to get the answer in cubic metres e.g 4x2x.75=6. There are 1000 litres in a cubic metre so that pond size will have a gross volume of 6000 litres. 

If you are old school then measure the same again in feet to give cubic feet e.g 13.1×6.5×2.5=212.9 then multiply it by 6.29 to get UK gallons. The answer is 1339 gallons, or about 6000 litres. But if you’re still not sure, head over to the easy to use swell pond volume calculator here

What pump do I need for my pond?

It depends what you need the pump to do. If you want a cascading fountain jet in the centre of the pond you need a fountain pump. If you want to filter the water you need a filter pump, and if you want to power a waterfall you could use either a fountain pump or a filter pump.

Conventional advice is that you need to turn over the total volume of your pond once every two hours, so a 5000 litre pond will need a 2500lph pump, and a 10,000 litre pond will need a 5000lph pump, etc. But most people opt for much larger models these days. A controllable pump offers the best of both worlds. 

What filter do I need for my pond?

There are two main types of pond filter – pressurised and gravity. A pressurised filter is a sealed unit which can be placed at the bottom of a waterfall, even in the ground up to its lid. The advantage with pressurised is that they are discrete and can be hidden, but they have their limits on very large ponds and may block frequently. 

Gravity-fed filters either have water pumped into the top of them where it cascades through a variety of filter media, or they are placed at the same height as the pond water level and water enters them via a bottom drain. The first type would need to be placed at the highest point in the system, so at the top of a waterfall and visible, or the second type, popular in purpose-built koi ponds, would need its own chamber next to the pond and some thought about design and placement. The advantage with large gravity filters is that they can cope with much more physical and biological waste.

So a pressurised filter for convenience on a small pond, and a box filter or gravity filter for large, dirty ponds. Tiny ponds can use an all in one pump and filter, combining pump, filter and UV in a small unit which sits in the pond. 

How much liner do I need for my pond?

Measure the maximum length and width of the pond in metres. Then measure the maximum depth, double it, (because liner goes down to the base and then up again,) and then add that to the length and width. So a 4x2x.75m pond would be 4+1.5(5.5) x 2+1.5 (3.5), or a 5.5×3.5 metre liner. Round it up a bit for overlap and you’ll need a 6x4m liner. But Swell can do all the calculations for you, for trouble=free liner selection here and don’t forget underlay for extra peace of mind!

Are my pump and filter too powerful for my pond?

You can’t really over filter a pond but you can have too much flow. Koi, goldfish and tench don’t like a lot of water flow whereas Orfe, Sterlet, Gudgeon, Rudd and Barbel do. If you have a very powerful pond pump try to offer an area of quieter flow for the fish to rest if they need it. Koi and goldfish aren’t adapted to be fighting strong currents 24 hours per day. 

You can have a pump which is too powerful for your UV, and too powerful for gravity-fed filters, however. Ultraviolet clarifiers (UVC) work by zapping water that flows past them with Ultraviolet light. Send water through the unit too fast and the contact time isn’t there to zap the green algae. So slower flow is always better with a UV. Gravity-fed filters work by settling particles out from the water by passing it slowly through a variety of filter media. Jet water in too fast and again it may just pass straight through without being settled out and caught like it should be.

If you have a powerful pump and need to keep it, you may need to upgrade your UV and gravity filter to cope with the extra high flow rate. 

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Jeremy Gay is an author and freelance aquatic specialist. A former editor of Practical Fishkeeping magazine, he offers a wealth of experience on all things aquarium and pond.


  • Avatar Chris Smith Posted 16/04/2020 at 1:46 pm | Permalink | Reply

    Hi I’m building a triangle shaped pond 5500 ltr and i want to keep koi . What’s size pump and filter do I need after to buy a complete kit . With. Built in uv etv

    • Avatar Jeremy Gay Posted 21/04/2020 at 9:18 am | Permalink

      Hi you have lots of filter options, and the main one is if you can accommodate a box style filter or if you want a compact pressurised filter. The Eazy Pod Complete by Evolution Aqua is built for koi and should be combined with a filter pump of about 5000lph. The uv is built in. If you need to go compact and pressurised then choose a pressurised filter model for 20,000 litres, as they halve then halve again their volume recommendation for koi. Choose the Oase Filtoclear Premium Set 20,000 for the best pressurised option.

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