Rapid pH Drop in Large Pond: Algae and Buffering

Situation

8000-gal pond, pH dropped from 7 to 6 in hot weather. Water slightly green. Good shade, NitraMax filter + UV.

Why pH Drops

Algae (even with UV) photosynthesize by day (consume CO2 → pH rises), respire at night (release CO2 → pH falls). Hot weather accelerates this cycle. Decaying algae also release acids.

Don't Chase pH

pH 6–11 is normal range. Rapid changes are dangerous. Stabilize with alkalinity (KH).

Fixes

1. Add crushed coral/oyster shell in filter bag → slowly raises KH/pH.
2. Add more plants: Water lilies, lettuce, hyacinth, cattails, iris – compete with algae, stabilize chemistry.
3. Weekly 10% water changes dilute acids.
4. Ensure UV bulb is fresh (replace annually).

Monitoring

Test KH weekly. Target KH > 100 ppm (5.6 dKH) for stability.