Why is my pond water green?

Pea-soup green pond water is usually caused by tiny algae suspended through the water. It is not the same problem as long strands of blanketweed.

Algae are a natural part of ponds, but they grow strongly when conditions suit them. Freshwater Habitats Trust identifies excess nutrients, especially nitrate and phosphate, as the shared cause behind green water, blanketweed and heavy duckweed. Common sources include tap water, soil or turf in the pond, bare soil washing in, and fish food and waste. The RHS also points to sunny ponds with little plant cover.

Treat the conditions rather than repeatedly changing all the water. Keep soil and fertiliser run-off out, use rainwater for topping up where possible, remove excess decaying material, and establish suitable plants without covering the whole surface. A fish pond may need a correctly sized filter and UV clarifier; a wildlife pond normally does not need either.

Check which green problem you have

Fill a clear jar with pond water and let it stand. Uniformly green water points towards suspended algae; water that clears in the jar while green strands remain in the pond points towards blanketweed or another plant. Look for recent inputs: a tap-water top-up, disturbed soil, uneaten fish food or lawn feed washed from the bank. Correct those before buying equipment. If fish are gasping or behaving abnormally, treat that as a welfare concern and contact an aquatic specialist or vet rather than waiting for the colour to improve.

Keep the jar out of children's reach and return the sample to the same pond.

Sources

• Freshwater Habitats Trust: frequently asked questions — accessed 14 November 2026.
• RHS: pond algae and blanket weed — accessed 14 November 2026.

More from Pond Setup Guide

Use the full green pond water guide and compare it with blanketweed. Algae, nitrate and UV clarifier are defined in the pond glossary.