Powdery Green Deposits After UV Clarifier: Not Pool Chemistry

Owner runs UV clarifier; water still green (not neon). Asks about powdery green stuff settling on rocks/plants. Answer provided is a pool water balance/Langelier Index essay—irrelevant to pond.

Correct Answer for Ponds

The “powdery green stuff” is dead algae (killed by UV) settling out. UV clumps single-celled algae into larger particles that filter or settle. It’s a sign the UV is working.

What to Do

• Increase mechanical filtration: Fine filter pads, filter socks, or settling chamber to catch the powder.
• Clean filter more often: Dead algae loads filters fast.
• Add a skimmer: Removes floating clumps before they sink.
• Be patient: Green water clears in days to weeks with proper UV + filtration.

Pool saturation index calculations don’t apply to biological pond systems. Focus on filtration of UV-killed algae.