Maintaining the clarity of koi pond water

Page Summary: The bane of so mnay pondkeepers lives... that green murky (maybe smelly) garden pond. You can change the water and then it suddenly comes back within a short space of time. Were talking of green murky (sometimes brown) water caused by suspended algae formation. The secret to eradicating suspended algae which is the cause of murkiness (brown or green) is to install both a biofilter and an UV (Ultra Violet) light called a clarifier, incorrectly sometimes as a sterilizer. By pumping water containing algae suspended in the water through a correctly sized UV light the tiny algae particles are forced to "clump" together and they can then be removed in the pond filter. Getting the right size of UV is the secret to success. Viresco is a natural alternative to remove algae in a fishpond.

Crystal Clear Clean Ponds

One of the great joys of keeping koi is being able to see them swimming around in the water. If your pond is full of murky water, then you wont see the fish clearly. This doesnt mean that the koi are necessarily unhappy, since water clarity and water quality dont always go hand in hand. In fact a crystal clear pond could be harbouring bacteria that could make your koi – and goldfish for that matter – quite miserable. Furthermore, as any experienced koi keeper will tell you, fry do much better in murky water, particularly when blanket weed algae provides a partial cover that they can nibble on.

You can only monitor the condition of your water by testing it regularly. And you can only maintain the condition of the water by fastidious filtering. This will also maintain the clarity of the water.

Fish pond filters

There are two sorts of basic pond filter, the mechanical kind and the biological type. While mechanical filters catch waste and debris, they dont remove toxic matter like harmful ammonia. Biological filters on the other hand are not really filters they are designed to purify pondwater. A secondary job is to remove solids. Algae is far too small to be removed by a filter unless first killed by UV light.

While there are different types of pond filters  – which in turn use different forms of filter media – it is very important to ensure that the filter can cope with the volume of water in your pond.

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You also need to be sure that the filter can deal with the volume of waste produced by your koi.

It also takes a bit of time for the biological action of a new filter to work effectively. Clean, new media dont trap microscopic particles effectively; so you will have to be a little patient in the early days.

Things that make pond water murky

Algae are a common cause of cloudy, murky water. While blanket weed forms on the surface of the water, other types grow suspended in the water, quickly producing green water. Algae thrive in hot weather and unless you keep checking the pond, you might find it turns green and murky overnight. A UVc installed as part of your filtration system is the best way to control green water caused by algae. Just be aware that UV tubes dont last forever and you will probably need to replace it annually.

Excess food in the pond water can also make it cloudy. This might be because the food has broken up as it enters the water, or because food dye has leached into the water. It could also be simply because you are overfeeding and the koi are not eating all the food.

Testing the water quality of koi ponds

If you own a swimming pool, chances are you test the water regularly and adjust it if the pH, total alkalinity and chlorine levels are not at the required levels. While the requirements for a fish pond are quite different to those of a swimming pool, it is just as important to test pond water regularly. Dont wait until you have a pond full of green water that is full of algae, or brown water that contains excessive koi excreta.

While you can take the water to a dealer to test a sample of the pond water for you, there are also excellent test kits that you can buy that will enable you to test:

Ammonia readings should always be zero, and the nitrate readings should be as low as is possible, also ideally zero. Ammonia and nitrate in the water will tend to make it cloudy and can affect your fishes appetites.

The ideal pH level for koi is between 6.8 and 7.5, which is virtually the same as the levels we aim for in our swimming pools. We measure pH according to a logarithmic number scale of zero to 14. If the water is neutral it will have a pH reading of 7. If the reading is over 7 the water is alkaline and if it is below 7, it is acid. In swimming pools a high pH will make water cloudy and scale will start to form on the walls of the pool. If it is too low, the plaster will start to etch and if there are metal fixtures in the pool these are likely to corrode from the excessive acidity. A low pH causes skin problems and eye irritations in humans. A low pH can damage the gills of koi, goldfish and other pond fishes. Serious swings in pH can kill them.

There are various ways to adjust the pH and general hardness (GH) of pond water, including the use of high calcium fish pond media (in your filter). Examples include media made from limestone or crushed oyster shells. There are also lots of proprietary products that, added to the water, will help to adjust and maintain the pH levels and get rid of ammonia and nitrates.

A pond aerator will ensure that there is enough oxygen in the pond water to help keep it clear and healthy.

Ultimately, a simple maintenance programme will ensure that your koi have crystal clear, sparkling water that is healthy as well.


On this site we dont pretend to be "Koi Crazy" despite the fact that you will see I refer to koi more often than not.

Make fish pond maintenance easy all year round ... Pond vacs, pond nets tools and protective clothing for cleaning your pond

Fish food and water treatments for pond and fish

Ready for Breeding Koi in Spring

In spring when loves comes around the koi will need somewhere to lay their eggs. In a natural environment eggs will adhere to submerged aquatic plants but in a koi pond there are not always plants because the koi tend to eat them.

Youll be amazed at how simple it can be to breed fish with the help of spawning brushes. Spawning BrushYoull need one mature female and at least two males.

Put the brushes one above the other at the edge of the pond. Spawning usually occurs in the early morning after a night of flurried activity and can take several hours. Most of the eggs will stick to the brushes which can then be taken out of the pond and put in filtered water of the same temperature. If the eggs are not separated from the adult fish they may be eaten.

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