Controlling Algae in a Small Pond with Goldfish and Mosquito Fish

Algae Overgrowth in a 300‑Gallon Pond

A pond owner in Northern California described a small pond (under 300 gallons) with goldfish, mosquito fish, a fountain pump, a biofilter with plastic balls, and a water lily. Despite barley pellets and bales, algae covered the surface daily and clogged the pump every other day.

Why Barley Alone May Not Work

Barley straw releases compounds that inhibit algae as it decomposes, but it works best when combined with other strategies. In a small, nutrient‑rich pond with limited plant coverage, algae can outcompete barley’s effects.

Effective Algae‑Control Strategies

• Add floating plants – Water hyacinth, water lettuce, or azolla shade the water and absorb excess nutrients. Check local regulations; some floaters are restricted in certain states.
• Increase filtration – A waterfall or larger biofilter improves mechanical and biological filtration.
• Reduce nutrient input – Avoid overfeeding fish; remove decaying leaves and debris.
• Consider a UV clarifier – UV sterilizers kill free‑floating algae cells and can clear green water quickly.

Local Resources

Contacting a local water‑gardening center or county extension can provide region‑specific plant recommendations and legal guidance.

Key Takeaway

Barley helps, but a combination of shading plants, adequate filtration, and nutrient management is usually needed for lasting algae control in small ponds.