Black Worms Clogging Waterfall: Blackfly or Caddisfly Larvae?

3–4 mm black worms anchor at one end, form balls/lines in running water, clog waterfall holes. Fish initially ate them but ignore now due to sheer numbers.

Identification

• Blackfly larvae (Simuliidae): Fan-shaped head, anchor with posterior sucker, thrive in fast flow. Match description.
• Caddisfly larvae: Build cases from silk/debris; smaller, less likely to form dense mats on bare rock.

Management

• Harmless: Indicate clean, oxygenated water.
• Fish can't reach them in waterfall crevices.
• Manual removal: Scrub off during maintenance; fish will eat dislodged larvae.
• No chemical control needed—they're beneficial filter-feeders.

They're a sign of a healthy stream-like microhabitat. Accept them or clean the waterfall periodically.