Pool store shelves are full of algaecides with varying active ingredients, concentrations, and claims. Choosing wrong can result in a pool that foams, stains, or fails to address the specific algae type you are treating. This guide covers the three main algaecide chemistries — polyquaternary ammonium (PolyQuat), copper-based, and traditional quaternary ammonium — with concrete guidance on when to use each.
Before diving into product selection, establish this fundamental concept: algaecides are supplements, not primary sanitizers. Most algaecides work as algistats — they inhibit algae growth and make it easier for chlorine to finish the job. A pool with zero chlorine will get algae regardless of how much algaecide is added. A pool with correct FC:CYA ratio will rarely need algaecide treatment at all.
The appropriate workflow is: treat with chlorine first (shock if active algae is present), then add algaecide after FC is in range as a follow-up and prevention tool. Adding algaecide to a green pool without shocking first is like applying weed killer without pulling the weeds.
Polyquaternary ammonium chloride (PolyQuat) at 60% concentration is the benchmark algaecide for pool service professionals. It is effective against green, yellow/mustard, and early-stage black algae as a preventive and supplemental treatment.
PolyQuat disrupts algae cell walls through electrostatic interaction. Algae cells carry a negative surface charge; PolyQuat's positively charged polymer binds to the cell wall and destroys membrane integrity, killing the cell. The high molecular weight of the "poly" (polymer) version means it is non-foaming — critical for pool use.
| Use Case | PolyQuat 60 Dose per 10K gal | Timing |
|---|---|---|
| Pool opening (prevention) | 16 oz | At opening, after shocking |
| Weekly maintenance | 4–8 oz | Once per week with FC in range |
| After algae treatment (follow-up) | 16 oz | After SLAM complete and FC returning to normal |
| Pre-storm prevention | 8–16 oz | Before expected heavy rain or bather events |
Copper sulfate and chelated copper compounds are highly effective algaecides, particularly against yellow/mustard algae that can resist standard chlorine doses. Copper ions penetrate algae cell walls and disrupt enzyme function.
Only copper-based products at 9–10% copper sulfate pentahydrate (or equivalent chelated copper concentration) provide reliable treatment-level efficacy. Products with 3–5% copper are maintenance-only — not treatment doses. Read labels carefully.
Copper in pool water stains. When copper concentration exceeds 0.3–0.5 ppm, it can precipitate onto pool surfaces as copper-based compounds. The result depends on the surface:
Traditional quaternary ammonium algaecides (10% concentration products, sometimes sold as "algae killer" at discount stores) are the oldest category. They work on algae cell membranes similarly to PolyQuat, but their low molecular weight causes severe foaming.
Foaming is a serious problem — it indicates surfactant activity in the pool, can interfere with skimming and filtration, and causes bather irritation. Traditional quats at 10% are largely obsolete for professional use. The only scenario where they appear is in discount store products or very old service contracts that haven't updated protocols.
Unless you are specifically prescribed a quaternary product by an equipment manufacturer or have a legacy reason, avoid 10% quat products and use PolyQuat 60 instead.
| Algae Type | Primary Treatment | Algaecide Type | Notes |
|---|---|---|---|
| Green (Chlorophyta) | SLAM (chlorine shock) | PolyQuat 60 post-SLAM | Most common; responds well to chlorine |
| Yellow/Mustard | High-FC shock + brush | Copper algaecide (9%+) | Clings to surfaces; retreats to walls when disturbed |
| Black/Blue-green | Brush aggressively + sustained SLAM | Copper + PolyQuat combo | Root penetration into plaster; very difficult to eradicate |
| Pink slime / water mold | Hydrogen peroxide (Burnout) in PHMB pools | PolyQuat for prevention | Not true algae — fungal/bacterial; chlorine solves in chlorine pools |
SplashLens tracks chemical treatments including algaecide type, dose, and date for every account. Know which pools have a history of algae problems and schedule preventive treatments before outbreaks occur. Free and offline.
Open SplashLens Free →PolyQuat 60 (60% polyquaternary ammonium chloride) is the best general-purpose algaecide for pool maintenance and prevention. It does not foam, does not stain, and is compatible with all sanitizer systems including salt and Baquacil. For yellow/mustard algae specifically, copper-based algaecides at 9–10% copper sulfate provide more effective treatment.
No. Algaecides are supplements to chlorine, not substitutes. Most algaecides work as algistats — they slow or prevent algae growth but cannot eliminate an established algae bloom alone. Chlorine shock is always required for active algae treatment; algaecide is added after chlorine to help prevent regrowth.
Yes, copper algaecide can stain pool surfaces if copper concentration in the water exceeds 0.3–0.5 ppm. Green, teal, or black staining on plaster, vinyl, and fiberglass is caused by copper precipitation. The risk is highest when copper algaecide is dosed too heavily or when pH drops below 7.0 while copper is elevated.
PolyQuat 60 is a liquid algaecide containing 60% polyquaternary ammonium chloride. Opening dose is typically 16 oz per 10,000 gallons; maintenance dose is 4–8 oz per 10,000 gallons weekly. Add directly to pool with pump running — no pre-dissolving needed.