Green Clean is a peroxide-based algaecide designed for ponds with koi, goldfish, trout, and other sensitive fish. Unlike copper-based algaecides, which can poison koi at concentrations as low as 0.15 ppm, Green Clean uses sodium carbonate peroxyhydrate that breaks down into hydrogen peroxide on contact with water. It kills algae through oxidation within 60 seconds of contact and releases oxygen as it works. Green Clean comes in two forms: Green Clean Granular for spot-treating string algae on rocks and waterfalls, and Green Clean FX Liquid for whole-pond treatment of planktonic and filamentous algae. Both are EPA registered and safe for fish, plants, pets, and wildlife when used as directed.
Why Copper Algaecides Are Dangerous for Koi
Most pond algaecides on the market use copper as the active ingredient. Copper is effective against a broad spectrum of algae but is toxic to koi at much lower concentrations than to most other fish.
Koi can be poisoned by copper at concentrations as low as 0.15 ppm, while goldfish tolerate up to roughly 0.3 ppm. The "safe" copper algaecide dose on most product labels is calibrated for general pond fish, not for koi. Trout and catfish are even more sensitive than koi.
Copper also accumulates in pond sediment over time. Repeated applications build up copper levels in the substrate, eventually poisoning the entire ecosystem. Soft water ponds (low alkalinity) are particularly dangerous because copper toxicity increases as alkalinity decreases.
For these reasons, koi keepers and serious water garden owners use peroxide-based algaecides like Green Clean instead of copper formulas.
How Green Clean Works
Green Clean's active ingredient is sodium carbonate peroxyhydrate. On contact with water, it breaks down into sodium carbonate (washing soda) and hydrogen peroxide.
The hydrogen peroxide oxidizes algae cell walls within 60 seconds of contact. Algae cells rupture, chlorophyll bleaches, and the algae dies. As the reaction completes, the byproducts break down into water and oxygen. The treatment leaves no chemical residue and actually adds dissolved oxygen to the pond.
Visible effects of treatment:
- Bubbling and fizzing on contact with algae
- Bleaching of green algae to white or tan
- Dead algae floating to the surface within 24 to 48 hours
- Algae mats breaking apart and dispersing
The same chemistry that makes Green Clean fast also makes it selective. Peroxide does not persist in the water column after the initial reaction, which means it kills the algae present at the time of treatment but does not provide long-term residual control. Most ponds require multiple applications through the season.
Green Clean Granular vs Green Clean FX Liquid
Two forms cover different problems. The Pond Shop carries both Green Clean Granular and Green Clean FX Liquid for koi pond use.
Green Clean Granular
Granular Green Clean is the right choice for visible string algae growing on rocks, waterfalls, streams, and shallow zones.
Best for:
- String algae attached to rocks and waterfall surfaces
- Filamentous algae mats on the bottom of shallow ponds
- Spot treatment of specific problem areas
- Water gardens and koi ponds with visible algae growth
Application rate: 1 pound treats 1,000 gallons of water.
Application steps:
- Apply in the morning hours of a calm, sunny day with water temperatures above 50°F
- Turn off water circulation pumps for waterfalls and streams
- Broadcast or sprinkle the granular product directly onto algae-covered rocks, stream beds, or surface mats
- Allow 20 to 30 minutes of contact time
- Turn water circulation back on
- Skim dead algae and organic matter from the surface as it rises
- For heavy growth, allow several days between repeat treatments
Sunlight and warm water accelerate the reaction. Cloudy days and cold water slow it down.
Green Clean FX Liquid
Liquid Green Clean FX is designed for whole-pond treatment of planktonic algae (green water) and dispersed filamentous algae throughout the water column.
Best for:
- Green water from planktonic algae
- Suspended filamentous algae in deeper water
- Whole-pond prevention and ongoing maintenance
- Spring pond openings when water is still cool
- Ponds where shutting off circulation is undesirable
Application rate for active algae treatment: 4 fl oz per 500 gallons of water.
Application rate for ongoing prevention: 2 fl oz per 500 gallons every 7 days.
Application steps:
- Mix the recommended dose with pond water in a bucket
- Pour the diluted solution into the pond in areas with active water movement (near fountains, waterfalls, or aerator outflow)
- Leave water circulation and aeration running for at least 48 hours after treatment
- Skim dead algae and organic matter from the surface
- Re-treat for heavy algae growth or regrowth, allowing several days between treatments
Liquid Green Clean FX does not raise pond pH, which is a meaningful advantage over the granular form for koi ponds where pH stability matters.
Safety Rules for Algaecide Use in Koi Ponds
Even with a koi-safe formula, technique matters. Three rules prevent the secondary problems that come from over-treatment.
1. Never treat more than 25% to 50% of the pond at once. Killing too much algae at one time releases all the nitrogen and phosphorus stored in the algae back into the water, plus the decomposition itself consumes oxygen. The combination can crash dissolved oxygen levels and cause fish kills even when the algaecide itself is safe. For heavy infestations, treat in sections over multiple weeks.
2. Run aeration during and after treatment. Dead algae consumes oxygen as it decomposes. Continuous aeration for at least 48 hours after treatment maintains dissolved oxygen levels through the decomposition window. This is critical in heavily stocked koi ponds.
3. Do not mix Green Clean with other algaecides or herbicides. Mixing chemical treatments can produce unpredictable reactions, including dangerous chemical incompatibilities. Treat with one product at a time and allow at least one week between different chemical applications.
4. Check pH before applying granular Green Clean. The granular form can raise pH up to 1 unit. Ponds running at pH 8.5 or higher should use the liquid FX formula instead, which does not affect pH.
5. Skim dead algae after treatment. Removing dead algae from the surface prevents the nutrients in the dying cells from being recycled into the water column to fuel the next bloom. Use a pond skimmer net within 24 to 48 hours of treatment.
What Green Clean Does Not Control
Honest expectations prevent disappointment.
- Chara (stonewort). Looks like algae but is technically a complex plant. Peroxide-based products are largely ineffective. Copper-based algaecides handle Chara more effectively, but cannot be used in koi ponds.
- Submerged rooted plants. Pondweed, hydrilla, milfoil, coontail. These are true plants, not algae. Use an aquatic herbicide instead.
- Floating plants. Duckweed, water lily leaves, water lettuce. Not affected by algaecides.
- Emergent plants. Cattails, phragmites, bulrush. Not affected by algaecides. Use a systemic emergent herbicide like Clearcast.
For mixed problems with both algae and rooted plants, use Green Clean for the algae and a targeted herbicide for the plants. Do not apply at the same time.
Long-Term Algae Prevention in Koi Ponds
Algaecide treats symptoms. The underlying cause of algae blooms is the nutrient load in the water. Long-term control requires reducing that nutrient load through:
- Continuous aeration to support beneficial bacteria that consume the same nutrients algae need
- Regular beneficial bacteria application to compete with algae for available nutrients
- Adequate mechanical and biological filtration sized for the koi load
- Limited fish feeding to reduce ammonia production
- Pond dye to block sunlight that fuels submerged growth
A pond running this prevention program needs Green Clean rarely, only for occasional spot treatment of stubborn growth on rocks and waterfalls. A pond without prevention will need Green Clean every few weeks all summer.
Frequently Asked Questions
Is Green Clean safe for koi? Yes, when used as directed. Green Clean uses sodium carbonate peroxyhydrate, which breaks down into hydrogen peroxide and oxygen on contact with water. It contains no copper, chlorine, or phosphates and is EPA registered for use in ponds containing koi, goldfish, trout, and other sensitive fish.
Why can't I use copper algaecides in a koi pond? Koi are highly sensitive to copper, with toxicity beginning at concentrations as low as 0.15 ppm. Copper also accumulates in pond sediment over time and reaches dangerous levels with repeated applications. Soft water (low alkalinity) ponds are particularly dangerous because copper toxicity increases as alkalinity decreases.
What is the difference between Green Clean Granular and Green Clean FX Liquid? Granular Green Clean is designed for spot treatment of string algae on rocks, waterfalls, and shallow zones. Apply directly onto visible algae with water circulation off. Liquid Green Clean FX is designed for whole-pond treatment of planktonic and dispersed filamentous algae. Apply diluted in pond water with circulation running. Liquid FX also does not raise pH, which the granular form can do by up to 1 unit.
How long does Green Clean take to work? Visible results within 60 seconds of contact with algae (bubbling and bleaching). Full algae die-off in 24 to 48 hours. Skim the dead material from the surface within 48 hours of treatment to prevent nutrients from recycling into the water.
Can I treat my whole pond with Green Clean at once? No. Treat no more than 25% to 50% of the pond at one time to avoid oxygen crashes from decomposing algae. For heavy infestations, treat in sections over multiple weeks with at least three days between treatments and continuous aeration throughout.
Will Green Clean kill my pond plants? No when used as directed. Green Clean targets algae specifically. Lilies, marginal plants, and submerged oxygenators are not affected at standard application rates. Avoid direct contact with the granular form on sensitive plant leaves during application.
Can I use Green Clean with beneficial bacteria treatments? Not at the same time. Algaecide treatment can stress beneficial bacteria colonies. Apply Green Clean first, wait at least 72 hours, then resume bacteria treatments. The bacteria will then consume the nutrients released by the dying algae and prevent the next bloom from establishing.
Need Help Choosing the Right Algaecide?
Call 800-527-9420 or email sales@thepondshop.com for help selecting between Green Clean Granular and Green Clean FX Liquid for your pond. Tell us your pond size, fish stocking, and algae type and we will recommend the right product and application program.
