How To Keep Koi Pond Water Clear

The ultimate aim for any fish pond owner is to be able to view and enjoy their fish. Here's the Swell UK guide to getting gin-clear pond water.

Fit a UV Clarifier

Ultra Violet Clarifiers are purpose-built to destroy the unicellular algae that cause green water in ponds. You cannot overdo the amount of UV you have on a pond but you can underdo it, and have too high a flowrate for it to be effective. So choose a large UVC model and make sure that your pump is pumping under the maximum flow rate for that size of UV. Run it 24 hours per day, change the bulb every 6-12 months depending on make and model, and keep the quartz sleeve clean too.

Still not sure which UVC you need? Select a pond filter with UVC built-in.

Trap the waste, flush the waste

You need really effective mechanical filtration if you want clear pond water which is free of particles. Use a large pond filter or several filters in combination and ensure that they have a sponge, static plastic media or a drum filter to capture those waste particles as they flow into the filter. The best mechanical filters are ones which are self-cleaning and can even automatically clean themselves, sending the dirt down the drain on a regular cycle. Remove the waste before it's broken down, and you'll have less nitrate and phosphate build-up, and less algae too.

Add some carbon

Activated carbon is a chemical filter media which soaks up dyes and discolouration from the water. Over time the water will turn yellow from biological processes, fish food, fish waste, leaves and dead algae, so add some activated carbon to your filter and it should return your clarity to that of tap water. Place it into a net bag first though so it doesn't escape from the filter and flow back into the pond.

Use a pint glass to analyse how clear your water is, by holding it up in the light.

Carbon can remove brown discolouration from the water

Add some bacteria

Bacteria are clever little things and have a solution for most fish-related problems. Add some bacteria aimed at clearing water and it should do exactly that, seeding the water with bacteria and enzymes which get to work reducing waste and polishing the water.

Change the water

Water changes have a myriad of benefits including lowering nitrates and phosphates, diluting hormones and helping to buffer pH and KH. Set a regular routine of water changes using water conditioner or a de-chlorinator, or better still set up a mains tapwater feed with a ball cock and a drain, to permanently trickle fresh water into the pond. A constant trickle of fresh, new water is best for growing fish to the maximum potential too.

Use a flocculant

Flocculants are liquid treatments that are poured into the pond to clear the water. They actually work by clumping particles of debris together, forming larger particles which then either fall to the bottom of still ponds, or are removed by filtration. They do work best in conjunction with mechanical filtration, and they actually make the water clarity look worse for a few hours before it gets better. But a dose of flocculant every now and then can really polish the water and help to make it look gin-clear.

Shade the pond

Few sunlit ponds enjoy clear water year-round as its too tempting a breeding ground for nuisance algae. If designing a pond place it in partial shade, build a pergola or fit a sail over the pond to protect it from the full glare of the sun's rays.