Identifying and Managing Brownish-Orange Algae on Waterfall Rocks

What Is That Rust-Colored Slime?

The brownish-orange film on waterfall rocks is typically a complex biofilm: a mix of brown and green algae, diatoms, bacteria, fungi, and trapped sediment. It's essentially the pond's "skin"—a natural, usually harmless community.

When to Be Concerned

If the coating is thick, smelly, or associated with poor water quality (high nitrates, low oxygen), it may indicate excess nutrients. Otherwise, it's mostly aesthetic.

Control Methods

• Physical removal: Scrub rocks during maintenance.
• Shade and competition: Plant watercress or other marginal plants in the waterfall to reduce light and absorb nutrients.
• Biofilm grazers: Snails and certain insects (like caddisfly larvae) naturally graze this layer.
• General algae control: Follow the keys—plants, low light, good filtration, beneficial bacteria, barley straw.

For more, see pond algae control.