Brown Film on Waterfall Rocks: Biofilm, Not Algae

That brownish slime coating waterfall rocks? It's not algae—it's a bacterial biofilm (iron bacteria, manganese oxidizers, or organic sludge). Here's how to manage it.

What It Is

• Microbial colonies feeding on dissolved organics, iron, manganese.
• Indicates: high organics, low flow spots, pH imbalance.
• Harmless to fish but unsightly.

Primary Fix: Stabilize pH

• Target pH 7.2–7.6.
• Test weekly; adjust gradually with pH buffers.
• Add pond peat (in mesh bag) to filter or stream—naturally stabilizes pH, reduces biofilm.

Secondary Measures

• Increase flow over rocks (adjust pump/nozzle).
• Scrub rocks monthly with stiff brush (no chemicals).
• Algaecide (pond-safe) can help lift heavy buildup for removal.
• Beneficial bacteria weekly—outcompetes biofilm formers.

Don't Over-Treat

Some biofilm is normal and beneficial (nitrification). Aim for control, not sterilization. If pH is stable and flow good, a thin brown coat is just a healthy pond patina.