Green Water in 90-Gallon Pond: Overstocking & Sunlight

90-gal pond, 12 goldfish (9 feeders + 3 fantails), full sun, green water 5 weeks despite filter cleaning, Pond Clear, algaecide. Larger pond same treatment stays clear.

Root Causes

• Severe overstocking: 12 fish in 90 gal = ~7.5 gal/fish. Minimum 20 gal/fish for goldfish.
• Full sun + high nutrients from fish waste = algae bloom.
• Small volume = unstable parameters, rapid heating.

Solutions

1. Reduce fish load: Rehome to 2–3 fish max.
2. Add shade: Floating plants (hyacinth, lettuce) cover 60% surface.
3. Upgrade filtration: Larger biofilter, UV sized for pond.
4. Barley straw: Mini bale in filter flow.
5. Partial water changes: 20% weekly.

The larger pond works because it has more volume per fish. Fix the stocking density first.