Clearing Green Water in a Koi Pond: Nutrient Control Is Key

A 1,000-gallon koi pond with 7 koi, 4 plants, and full sun develops persistent green water (planktonic algae) despite string algae control. The owner hesitates to use rock salt, seeking a safe, effective solution.

Why Green Water Persists

Green water is caused by free-floating single-celled algae that thrive on excess nutrients—primarily nitrate and phosphate. Weekly string-algae treatments don’t address the dissolved nutrients fueling the bloom.

Effective Strategy: Comprehensive Nutrient Management

1. Full water test: Get a 16-parameter lab analysis (nitrate, phosphate, KH, GH, iron, etc.).
2. Identify excess nutrients: Compare results to target ranges for algae control.
3. Reduce inputs: Cut feeding, remove decaying leaves, divert runoff.
4. Increase export: Add more plants (especially fast-growing marginals and floaters), upgrade filtration, perform regular water changes.
5. Consider a UV clarifier: Properly sized (2–3 watts per 1,000 gallons) for green water control.

Rock salt is not a reliable algae remedy and can harm plants at effective doses. Focus on the nutrient budget; clear water follows balanced chemistry.