Algaecide is insurance, not a rescue

The three chemistries, and which one belongs in a vinyl pool

For the pools this site covers, that table has one answer. A vinyl liner is exactly the surface copper marks, and our liner stains guide is explicit that metal stains do not respond to the chlorine tablet test and cannot generally be lifted once they have set into the vinyl. Foam from a quat product is less serious but it is the most common avoidable cause of a foaming pool — our pool foam guide covers it.

Polyquat costs more per dose and avoids both. On a pool holding a few thousand gallons that price difference is small in absolute terms, and it buys you out of the two failure modes.

What algaecide is actually for

Prevention. Dosed at the label rate into water that is already clear and sanitized, it makes a bloom substantially less likely during the conditions that cause one — a heatwave, a stretch of heavy use, a week you were away. That is a real benefit and it is cheap insurance in high summer.

It is not a rescue. Once algae has established, the biomass in the water is beyond what a preventive dose addresses, and the chlorine demand is high enough that the algaecide is competing with the bloom rather than killing it. The sequence that works is the one in our algae guide: brush every surface, shock heavily at dusk with the pump running, filter continuously until clear, vacuum the dead algae, and only then dose a preventive algaecide so it does not come back.

Doing that in the wrong order is the single most common way people spend money on this category and get nothing for it.

The mustard exception

Yellow-brown algae that clings to shaded walls and returns within days of being brushed off behaves differently — it tolerates chlorine levels that ordinary green algae does not, and it reinfects from floats, nets and vacuum heads that were not treated. Our mustard algae guide is the specific procedure, and the algaecide step in it is still the last step rather than the first.

What prevents algae better than algaecide does

Three things, in order of effect, and two of them are free.

A chlorine level that does not sawtooth. Most blooms start in the mid-week trough between Saturday doses. A floating dispenser meters tablets steadily at the published rate of two three-inch tablets a week per 10,000 gallons, and it prevents more algae than anything in a bottle — our chlorine floater roundup covers the hardware.

Brushing. Biofilm establishes on walls well before it is visible as green. Weekly brushing, water line included, removes it while it is still nothing — our brush roundup covers what to buy.

Watching cyanuric acid. A pool whose stabilizer has climbed through a season of tablets has sluggish chlorine that reads well and acts poorly, which is the condition algae exploits. Our cyanuric acid guide covers the fix, which is dilution.

Dosing, and the mistake worth avoiding

Follow the label rate for your actual gallons, not for the pool you wish you had. On the volumes this site covers — 1,485 to 6,981 gallons — the dose is a fraction of what a label sized for 10,000 or 20,000 gallons implies, and overdosing a quat product is how a pool ends up foaming for a week.

Add it with the pump running so it distributes, and add it after the water is clear rather than while you are still fighting something. Our weekly schedule puts a preventive dose on a monthly rhythm in high summer, which is the right cadence for most seasonal pools.