How to clear green pond water

Green water affects many ponds and spoils the view of pond fish. Here are the answers to some Frequently Asked Questions on Green Water.

What is green water?

Green water is made up of millions of tiny algae cells all floating around in the water. They are green because of the chloroplasts inside them which photosynthesize and turn sunlight into energy. Just like plants.

What causes green water?

Spores are probably airborne, but any body of water can turn green if there are sufficient light and nutrients. In the garden pond it is caused by open water being exposed to sunlight, and will be most prolific in hot weather with prolonged hours of daylight, and lots of nutrients like nitrates and phosphates from fish.

Is green water harmful to fish?

A little algae can be good for fish colouration and overall health. It can also help to disguise them from predators. But too much green water on a brightly lit day can produce too much oxygen, resulting in deadly embolisms on the fish. Predators not being able to see the fish also means that you can’t see them either, so it’s difficult to do a headcount and make sure that they’re all there and healthy.  

How can I clear Green water?

The quickest and easiest way to combat green water is to fit a pump, pond filter and Ultra Violet Clarifier. Pond water is pumped through the UVC where ultraviolet light damages and disrupts algae that pass through it. The algae cells are then caught in the mechanical section of the filter, and water quickly clears. Fit the right sized pump, filter and UVC and you can have clear water year-round. Just remember to fit a brand new bulb every spring, and clean or replace the quartz sleeve. UVC is the best and most reliable way to clear green water.

An Ultra Violet Clarifier will clear a pond of green water and keep it clear

Can I use liquids?

Greenwater can be cleared with a liquid treatment. Known as flocculants, green water treatments clump together the tiny cells into bigger lumps where they either sink down through the water or become large enough to be trapped and removed by a pump and filter. Flocculants can be highly effective, clearing water within hours of being administered, but you would need to use them several times a year, as and when the green returned. This could equal the cost of buying a UVC. Liquid green water treatments work best in combination with a fine mechanical pre-filter, to remove all those clumped together particles in one pass.

Is there a natural way?

Sometimes green water clears on its own. It blooms where nutrient levels are high, and can crash and disappear once those nutrients are used up. Changing water can make green water worse, as new nutrients are introduced from the tap water and feed the next bloom. Green water is rare in wintertime when water is cold and daylight hours short, and heavy planting will both help consume excess nutrients and shade much of the pond, cutting down available light. 

Water fleas and Swan mussels are nature’s green water filters, but water fleas are also predated by fish and removed by filters, so they would only work in wildlife ponds without fish and filters.

Are some ponds more prone to green water than others?

Yes, duck ponds are the most prone because ducks foul the water with their droppings, and eat plants. Shallow ponds in full sun are also very prone to it. Deep, cool, shaded ponds are the least prone, but all types of pond can be permanently kept clear of green water with enough filtration and a sufficient Ultra Violet Clarifier.     

Posted by on

Jeremy Gay is an author, lifelong fishkeeper, and aquatic specialist. He's a former editor of Practical Fishkeeping Magazine, UK editor at Reefbuilders, a former aquatic store manager, and has collected fish in Sri Lanka and the Amazon. He's been on tv and radio, contributed to Koi Carp and Gardeners World magazines, been a product tester, a judge, and a product developer. Jeremy is here to guide and advise you on all things tropical, pond and marine, from set-up to stocking, health, feeding to breeding, as well as solving many common fishkeeping problems along the way.


  • DEREK DENOVELLIS Posted 22/04/2020 at 2:53 pm | Permalink | Reply

    my pond is 4mtrs x 2mtrs and 1 mtr deep so what size fillter do i need please

    • Jeremy Gay Posted 28/04/2020 at 9:49 am | Permalink

      Hi there you have a maximum volume of 8000 litres. Choose a pump, filter and UVC kit suitable for ponds up to 8000 litres, or 16000 litres, if you intend to keep koi. There’s a useful blog explaining the difference between pressurised and black box filters here.

Add your comment

* Required fields