Green water commonly involves suspended algae. Nutrients, sunlight, filtration, and feeding all deserve attention. Water color alone does not tell you whether your koi are safe.
Identify what you are seeing
Green water differs from long strands attached to surfaces, brown sediment after rain, and cloudy water following a disturbance. Take a photograph in daylight and note whether the change was gradual or sudden.
Check fish and water first
Unusual fish behavior, gasping, or a failed pump needs prompt qualified attention. Record water-test results rather than relying on appearance. Avoid adding multiple treatments while the cause is still unclear.
Look for ongoing inputs
Review feeding, fish numbers, fallen leaves, and runoff entering the pond. A cleanout may remove accumulated debris without addressing what keeps feeding the problem. Filtration should be appropriate for the actual pond and fish load.
Choose a measured response
Ask a pond professional to inspect circulation, filter maintenance, and any UV equipment against its instructions. Treatments must be labeled for the specific use and compatible with fish and plants. Never use a fish-free fountain product in a koi pond by assumption.
What to record before a visit
Collect the pond dimensions, fish count, feeding routine, filter and pump models, treatment history, and recent test results. Include whether runoff reaches the pond after storms. This gives a clearer starting point than simply requesting an algae treatment.
Explore koi pond maintenance or check referral availability. This guide is general homeowner education, not an on-site diagnosis or emergency service.
Manufacturer guidance applies to the relevant equipment; always follow your own product manual. Links do not imply endorsement or affiliation.