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. Green water can make pond water look like pea soup but its actually one giant, spontaneous algae bloom.
What causes green water?
Green algae 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 waste.
A new pond will nearly always go green if filled with tap water not filtered by an ultraviolet clarifier (UVC.)
Is green water harmful to fish?
A little green algae can be good for fish colouration and overall health. It can also help to disguise them from predators. But too much green water in a fish pond 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 pond 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 pond 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.
Can I use liquids?
Green pond water 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 as dead algae 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.
Beneficial bacteria can be added in liquid, gel or powdered form, which once established won't kill green water and other nuisance algae outright, but it can help to clean the pond and cleanse pond water of excess nutrients caused by too much fish food.
Is there a natural way?
Sometimes green pond 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 pond plants like water lilies, floating plants and aquatic plants like oxygenators 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. Well established, heavily planted, lowly stocked ponds rarely suffer from green water, and the clarity of water is often an indication of a healthy pond.
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 pond 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.
Is Duckweed green water?
No, Duckweed are actually tiny floating plants which sit on the surface, not algae that floats around actually in the pond water. A surface skimmer is the only effective treatment for Duckweed and other smothering floating pond plants like Azolla.