Persistent Green Algae in Large Koi Pond: Filter Maturity and Maintenance

Pond Profile

12×15×5 ft (~6,700 gal), rubber liner, 2 large foam filters with 25W UV each, 40 large goldfish + 10 large koi. Green water persists despite chemicals, water changes, cleaning.

Root Causes

1. Immature biofilter: Foam filters cleaned too often → loss of nitrifying bacteria. Need separate mechanical (clean often) and biological (clean rarely) stages.
2. Insufficient UV: 50W total for 6,700 gal = 7.5 W/1000 gal. Target: 3 W/1000 gal minimum, but flow rate must allow 1-sec exposure. Check UV flow rating.
3. High fish load: 50 large fish = heavy nutrient input. Reduce stocking or increase filtration.
4. No aquatic plants: Floating plants died; they compete with algae for nutrients.

Corrective Plan

1. Restructure filtration: Vortex/sieve for solids → biological media (Japanese mats, Kaldness, moving bed) → UV (replace bulbs annually).
2. Run filter 24/7; turn over pond volume every 3 hours.
3. Add plants: Water lilies, submerged oxygenators, marginal plants in baskets.
4. Weekly 10% water changes spring through fall.
5. Spring startup: Add bottled bacteria, run UV 24/7.

Expectation

With mature biofilter, proper UV, and plants, green water clears in 2–4 weeks.