Black Water in a Koi Pond: Test pH and Nitrite First

A multi-year koi pond suddenly turns black. Cleaned twice yearly, minimal sludge. Owner asks cause and fish risk.

Immediate Diagnostics

1. Test pH: Target 6.8–7.8.
2. Test nitrite: Must be 0 ppm.

Likely Causes & Fixes

• Low oxygen at depth: Add bottom-diffused aeration (air pump + diffuser stones).
• Organic buildup: Add beneficial bacteria (e.g., Microbe-Lift PL).
• Tannins from leaves: Activated carbon removes color.
• Algae die-off: Sudden nutrient drop crashes bloom.

Black water often signals anaerobic conditions at the bottom. Aeration and bacteria are the first-line response. Test before treating.