Does Pool Algaecide Actually Work?
Short answer
Algaecide works well as a preventive backup but poorly as a cure. It cannot clear an established bloom, which requires chlorine at shock level. Polyquat 60 is the most useful type for routine prevention because it does not foam or stain.
Algaecide works, but not for the job most people buy it for. It is a preventive backup that helps stop a bloom from starting. It will not clear a green pool. Chlorine held at shock level does that, and no algaecide on the shelf is a substitute for adequate sanitizer.
What the different types actually do
Polyquat 60 (poly[oxyethylene dimethyliminio], often labeled PolyQuat or WSCP). The most useful type for routine prevention. It does not foam, does not stain, and works on a broad range of algae. It is the most expensive per dose and the one we generally recommend.
Quaternary ammonium (quats). Cheaper and effective, but it foams noticeably, especially with any water feature or aggressive returns. It also adds to chlorine demand.
Copper-based (chelated copper). Genuinely effective, including on mustard algae, but copper stains plaster and can turn light hair green if levels get away from you. Use only with careful balance and never in a pool with a metal problem already.
Sodium bromide. Not really an algaecide on its own. It converts to hypobromous acid when chlorine is added, and is used for stubborn mustard algae. Effective but it turns your pool into a bromine pool temporarily.
Why it will not clear a green pool
The dose required to kill an established bloom with algaecide alone would be far higher than the label allows, and in the case of quats you would produce a foaming mess. Algaecide is not a sanitizer, so it does nothing about the bacteria that come with an unsanitized pool. It also cannot filter out dead algae, which is what actually takes the days.
If your pool is green, the sequence is: balance pH to 7.2 to 7.4, brush, raise free chlorine to about 40 percent of your cyanuric acid level, hold it until overnight chlorine loss falls under 1 ppm. Add algaecide afterward, as maintenance, once the water is clear.
When algaecide is genuinely worth it
- As weekly insurance in a pool that runs on the edge of adequate chlorine
- Before you leave town for a week or two
- Ahead of a storm or a stretch you know you will miss
- After clearing a bloom, to reduce the odds of a repeat
- In pools with heavy shade, poor circulation or a lot of organic debris
Dose on the schedule the label gives, usually weekly or biweekly for polyquat. Overdosing does not add protection.
The honest verdict
Algaecide is a supplement, not a strategy. Correct free chlorine relative to CYA, pH at 7.4 to 7.6, adequate pump runtime and a clean filter prevent algae far more reliably than any bottle. If you find yourself relying on algaecide to keep a pool clear, something in the chemistry or circulation needs fixing.
Where it earns its keep in the Keys
Key West is one of the places where a preventive algaecide is more defensible than average. There is no cold season, so algae pressure runs all 12 months, and our UV strips chlorine fast enough that a pool can drop below its algae-holding threshold between service visits. For vacation rentals and second homes that sit unattended for stretches, a polyquat dose is cheap insurance against arriving to a green pool.
It is still insurance and not a plan, so fix the chlorine level first and let the algaecide cover the gaps.
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