Filamentous algae in Florida ponds: why it keeps coming back

Filamentous algae — the bright green stringy mats that float on the surface and cling to anything in the water — is the most common complaint we get on Florida residential and HOA ponds.

Why it blooms

Algae need three things: sunlight, warmth, and nutrients. Florida supplies the first two for free. The variable you control is nutrients.

The dominant nutrient sources on a typical Central Florida pond:

• Lawn fertilizer runoff from properties draining toward the pond
• Decaying aquatic vegetation that wasn't removed last season
• Septic seepage on properties pre-1990 that haven't converted to municipal sewer
• Waterfowl — a flock of resident geese or ducks contributes substantial phosphorus
• Stocked fish that are over-fed or over-stocked

Why algaecide alone fails

Copper-based algaecides kill the visible mat. The mat then sinks, decays, and releases the phosphorus it absorbed back into the water column. Two weeks later, you have a fresh bloom — fueled by the dead mat from the last treatment.

The actual fix

1. Aeration. Sub-surface diffused aeration runs 24/7 and prevents the oxygen crashes that follow algae die-offs.
2. Buffer plantings. A 3–6 ft strip of native shoreline plants (pickerelweed, arrowhead, duck potato) intercepts nutrients before they reach the water.
3. Fertilizer setbacks. No fertilizer within 15 ft of the waterline. Educate the neighbors.
4. Mechanical mat removal. When a bloom does occur, physically remove the mat instead of killing it in place — that pulls the nutrient load out of the system.
5. Targeted algaecide as a last resort, never as a routine.

Realistic timeline

A pond with a chronic algae history will take 12–18 months of combined treatment before the bloom pattern breaks. The first six months will still see blooms; the second six should show shorter, smaller events; year two should be a stable shoreline.