Clearing Murky Pond Water: Sand Filter Conversion & Fish Selection

Diagnosing the Green Water

Your 800–900 gal stone pond with 1 HP sump pump and waterfall but no filtration is a classic "green water" setup. Sunlight + nutrients (fish waste, no biofiltration) = algae bloom. Clear water in winter? Low light, cold temps, dormant algae.

Repurposing Your Pool Sand Filter

Yes, the sand filter can work—but not as-is. Convert to a stratified biofilter:

1. Layer (bottom to top): coarse gravel → pea gravel → coarse sand → filter sand.
2. Pump water up through bottom, down through layers (upflow) or down through top (downflow). Upflow resists clogging.
3. Seed with beneficial bacteria (bottled or from established pond).
4. Run 24/7. Backwash weekly initially, then monthly.

This provides mechanical + biological filtration. Add a UV clarifier (11–18W for your volume) after the filter for green water control.

Fish Selection: Cold-Water Only

Your unheated pond limits you to cold-water species. Koi & goldfish are correct choices. "Tropical" fish (cichlids, plecos, etc.) require 75°F+—heating 900 gal outdoors is cost-prohibitive.

For visible, vibrant, large cold-water fish:

• Shubunkins (calico, 12–14", hardy)
• Sarassa comets (red/white, 12–14")
• Orfe (golden/blue) — need >500 gal, very active
• Koi — if filtration handles bioload

Resources

Books: The Pond Doctor (Helen Nash), Koi Health & Disease (Dr. Erik Johnson). Websites: koipondinfo.com, pondmarket.com.