String Algae Control: IonGen and Other Solutions

String algae — those long, hair-like green strands that cling to rocks, waterfalls, and pond edges — is the second most common algae complaint after green water. Unlike suspended algae that clouds the water, string algae attaches to surfaces and can grow several feet long, clogging pump intakes and ruining the clean look of your waterfall.

Understanding String Algae

String algae (filamentous algae) is a natural part of every pond ecosystem. A thin layer of algae on rocks is actually healthy — it provides food for snails and some fish species and indicates a functioning biological system. The problem arises when string algae grows aggressively, overwhelming surfaces and choking water flow.

Good to know: A thin green coating on rocks and gravel is normal and beneficial. Only take action when string algae forms long strands, thick mats, or begins clogging equipment.

IonGen Electronic Algae Control

The IonGen system is one of the most effective tools for string algae control. It uses a low-level electronic probe installed in the pond's water flow path to release copper ions into the water. Copper is a natural algaecide that inhibits algae growth without harming fish, plants, or beneficial bacteria at the concentrations the IonGen delivers.

String algae control iongen — practical guide overview — String algae control iongen

How It Works

A control panel mounted near the pond adjusts the copper ion output
A probe with replaceable copper bars sits in the waterfall or stream flow
As water passes over the probe, microscopic copper ions dissolve into the flow
Copper concentration is maintained at levels toxic to algae but safe for fish (typically 0.25 ppm)

Pros and Cons

Pros: Set-and-forget operation, no recurring chemical purchases, effective against both string and suspended algae, safe for fish and plants.

Cons: Initial cost ($250 to $350), requires probe replacement every one to three seasons ($40 to $60), must monitor copper levels with test strips to avoid overdosing.

String algae control iongen — step-by-step visual example — String algae control iongen

Copper and snails: Copper is lethal to snails and other invertebrates at the concentrations used for algae control. If your pond has snails or freshwater mussels you want to keep, do not use the IonGen or any copper-based treatment.

Other Control Methods

Manual Removal

The simplest approach: physically pull string algae off rocks and surfaces. Use a toilet brush (dedicated to pond use), a stick, or your hands. Regular weekly removal prevents massive buildup. Compost the removed algae — it makes excellent garden fertilizer.

Beneficial Bacteria

Weekly doses of concentrated beneficial bacteria outcompete algae for nutrients. Products like Aquascape SAB (String Algae Buster) are formulated specifically for filamentous algae. Apply directly to affected areas and to the biological filter for best results.

Hydrogen Peroxide Spot Treatment

Standard 3 percent hydrogen peroxide applied directly to string algae clumps kills them on contact. Use a spray bottle for above-water surfaces and pour measured amounts (1 cup per 500 gallons) for underwater treatment. This is a spot treatment, not a pond-wide solution.

Barley Straw

Decomposing barley straw releases humic acids that inhibit new algae growth. Place mesh bags of barley straw in the stream or near the waterfall in early spring. Allow 4 to 6 weeks for the straw to begin decomposing before expecting results. Preventive only — it does not kill existing algae.

String algae control iongen — helpful reference illustration — String algae control iongen

Best strategy: Combine methods. Use IonGen or beneficial bacteria as the baseline, manually remove any outbreaks, and add barley straw as a preventive in spring. This layered approach provides the best control.

Prevention Fundamentals

String algae thrives on excess nutrients and sunlight — the same conditions that feed all algae types. Long-term prevention focuses on:

Maintaining adequate plant coverage (40 to 60 percent of surface area)
Not overfeeding fish
Removing organic debris promptly
Keeping the biological filter clean and functional
Running the pond pump 24/7 for continuous filtration

For green water (suspended algae) solutions, see our separate green pond water guide.