Why is my pond water green?

Green water that looks like pea soup is algae floating in the water as single cells. It is not the same thing as the hair-like strands on the liner. The RHS describes both on its algae and blanket weed page, and the fix depends on which one you have. Strands are blanketweed.

Why it happens

Those cells photosynthesise. They do well in sun, in warm water, and when the water is rich in nutrients. Nutrients come from fish waste, excess food, soil in the pond, leaves, and tap water used for topping up. A brand-new pond often goes green while it settles. A wildlife pond with no fish and plenty of plants often clears without a machine. A stocked goldfish pond in full sun often does not.

The RHS says there are no chemical treatments or weedkillers approved for home gardeners to control pond algae and blanket weed. Skip the bottle that claims to cure a green pond in a weekend. Some products only settle dirt for a short time. They are not a substitute for shade, planting and, where you have fish, a filter that is the right size.

What actually helps suspended algae

• Shade. The RHS suggests siting a new pond with some shade, not in full sun all day, and a depth of 60–75 cm where you can, because shallow water warms fast. On an existing pond, waterlily cover of about a third to a half of the surface is the June suggestion.
• Fewer nutrients. Do not overfeed fish. Top up with rainwater. The RHS says tap water should be a last resort, added in small amounts if you must, because of the nutrients and the temperature shock. See topping up.
• A filter and a UV clarifier, if you have fish. The RHS says a UV clarifier makes the floating algae clump so the filter can catch them. The lamp does nothing for algae that never passes through it. Size it from the UV guide, and size the filter from the filter guide. Hozelock says a visible change can take days and a clear pond can take six to eight weeks.
• Leave wildlife ponds alone if you can. Adding a UV and a pump to a pond with no fish changes the habitat. The RHS does not ask you to filter a wildlife pond.

What not to do

Do not empty the pond and refill it with tap water. You throw away the animals and refill with the nutrients that fed the algae. Do not keep switching the biological filter off. The RHS says the bacteria can die if it is off for 24 hours or more.

If fish are gasping at the surface, that can be a lack of oxygen, which green water and hot weather make worse. This site cannot tell you why a particular fish is in trouble. Ask a vet or an aquatic specialist the same day. The RHS suggests a bubble fountain in hot, humid weather to add oxygen, especially if you keep fish.

Will a bigger pump clear green water?

Only if the water then spends long enough in a UV clarifier and a filter that can catch the clumps. A pump on its own circulates the soup. PondXpert says the pump does not filter anything.

The pond was clear and went green after a week of sun. Is that blanketweed?

Look closely. If the water itself is cloudy green and you cannot see the fish, that is suspended algae. If you can lift long green hair out on a stick, that is blanketweed, even if the water between the strands is clear. You can have both.