What PoolBoss Says
Chlorine kills algae; algaecide only prevents it or backs up a treatment, so pick by side effect rather than strength. Polyquat is the safe default - no staining, no foam, highest cost. Copper works but can stain plaster and hair. Quats are cheapest and foam if overdosed. A well-chlorinated pool usually needs none.
The bottle sits on the shelf at every supply house, it is cheap enough not to argue about, and the label says it kills algae. Then you pour it into a green pool and almost nothing happens, or you dose a clear pool every week for two summers and hand the customer a stained waterline. Both outcomes come out of the same misunderstanding about what the product is actually for.
Algaecide is a preventative and a supplement. It is insurance on a pool you cannot watch closely, not a treatment for a pool that has already turned. What follows is what each class does, what it costs you in side effects rather than in dollars, and the handful of pools on a route where a preventative dose genuinely earns its place.
At a glance
Key takeaways
- Chlorine does the killing. Algaecide prevents algae from establishing and supplements a treatment; it never clears a bloom on its own.
- Default to polyquat. It costs the most at $25-$40 a quart and is the only class with no stain, no foam, and no added chlorine demand.
- Treat copper as a deliberate exception, not a routine dose. Above roughly 0.2-0.3 ppm it stains plaster and light hair, and it does not leave the water on its own.
- Skip the cheap quat on customer pools. It foams when overdosed and consumes the chlorine you are relying on.
- Dose preventatively only where chlorine loses control between visits: heavy shade and organic load, every-other-week pools, vacant properties, and the week after a recovery.
- Judge the product on repeat algae calls at the same addresses, not on how the water looks the day you poured it.
- If a pool needs algaecide every season, the real problem is circulation or a maintenance chlorine level too low for its cyanuric acid.
What kind of algaecide should I use in a pool?
Use polyquat unless you have a specific reason not to. It is the one class with no meaningful side effect: it does not stain, it does not foam, and it does not add to chlorine demand the way a cheap quat does. The tradeoff is price - polyquat typically runs $25-$40 a quart against $10-$20 for a quat, which is why the cheap bottle keeps ending up on trucks. None of the three will rescue a bloom either, so if the water is already green, chlorine and a brush are what clear it.
The three classes on a supply house shelf are not different strengths of the same product. They are different chemistries with different failure modes, and picking between them is mostly a question of which side effect you are willing to own on a customer's pool.
- Polyquat (polyquaternium): a polymer that keeps algae from establishing and clarifies the water slightly. No stain, no foam, no added chlorine demand. Highest cost, roughly $25-$40 a quart, with maintenance doses usually in the 2-4 oz per 10,000 gallons range.
- Copper (chelated copper compounds): genuinely effective, and it persists in the water long after chlorine has burned off. That persistence is the whole problem - copper does not leave on its own, and above roughly 0.2-0.3 ppm it starts showing up as stain.
- Quat (quaternary ammonium): the cheapest bottle and the weakest preventative. It is a surfactant, so it foams when overdosed, and it consumes chlorine rather than sparing it.
- Sodium bromide is sold alongside these but is not an algaecide in the same sense. It converts to bromine when you shock, and it exists almost entirely for one job: black algae.
Algaecide does not kill a bloom, and treating it like it does costs you a visit
The doses on the label tell you what the product is for. A maintenance dose is measured in a few ounces per 10,000 gallons, which puts a trace concentration into the water - enough to stop a few cells from getting a foothold in water that is already sanitized. An established bloom is billions of cells plus the organic load they came with, and that load carries a chlorine demand a trace polymer cannot touch.
So the bottle poured into a green pool disappoints for a reason that has nothing to do with the brand. It was formulated to prevent, and prevention and treatment are different jobs. What clears a bloom is free chlorine held at shock level long enough to hold there without falling back, plus brushing to break the colony open, plus filtration to pull the dead algae out. That sequence costs chemicals and a return visit; a $20-$40 bottle of algaecide costs $20-$40 and buys you a second trip.
The version of this that actually hurts is the pool that keeps coming back. A yellow bloom on a shaded wall is not a chlorine failure so much as a tolerance problem, and the mustard algae treatment that actually holds turns on a higher chlorine hold and sanitizing the gear in your truck, not on which bottle you added afterward. Reach for an algaecide there as a follow-up once the water is clear, if at all.
The three classes side by side
Read this table by the side-effect column rather than the cost column. On a route you carry the liability for a stain long after you have forgotten which bottle saved you $15.
| Class | What it actually does | Main side effect | Typical cost | When it earns its place |
|---|---|---|---|---|
| Polyquat | Prevents algae establishing; mild clarifier | None significant at normal doses | $25-$40 a quart | Default preventative on pools you cannot get back to quickly |
| Copper | Kills and keeps killing; persists in the water | Stains plaster, fittings, and light hair | $20-$30 a quart | A pool that came back despite a correct chlorine hold |
| Quat | Weak preventative; acts as a surfactant | Foams if overdosed; adds chlorine demand | $10-$20 a quart | Budget prevention on a pool you see weekly |
| Sodium bromide | Converts to bromine on shock; not a standing preventative | Raises bromine demand for days after | $10-$20 a treatment | Black algae, as part of a shock sequence |
Copper is the one that leaves a mark
Copper works, and that is exactly why it keeps getting overused. It stays in the water after chlorine has burned off, so a weekly dose accumulates with nothing to remove it except dilution or a drain. Above roughly 0.2-0.3 ppm, copper starts depositing on plaster as grey-blue or greenish staining, usually along the waterline and in the shaded low-flow corners where it settles. It also turns light hair green, which is the version of the problem a customer notices immediately.
The staining shows up on a delay, which is what makes it a route problem rather than a chemistry problem. A tech running 38 pools around Tucson and Oro Valley inherits an account where the previous company dosed copper weekly through two summers. The water is clear, the readings are perfect, and there is grey-blue staining along the waterline that no amount of brushing lifts. The customer has met exactly one pool company recently, and it is not the one that caused it. Removing it means a drain and an acid wash, several hundred dollars the owner did not budget for, and even then the deepest staining does not fully lift.
There is one place copper genuinely earns the risk. A colony with a protective layer that shrugs off ordinary shock is a different organism than a green bloom, and black algae and why it is the exception is the case where a copper product added after hard brushing is the tool that finishes the job. Use it there, deliberately, and tell the customer what the tradeoff is before you pour it.
Where a preventative dose genuinely pays on a route
A preventative earns its cost on pools where your chlorine is going to lose control between visits. That is a short and specific list, not a route-wide policy. On the same 38-pool Tucson route, 11 pools sit under mesquite and get heavy shade plus a constant organic load, and after a monsoon storm week those are the pools that go yellow. A polyquat dose on those 11 after a storm is roughly $4-$6 of product each against a recovery visit that costs an hour and a return trip.
The cases worth dosing are consistent across routes: pools with heavy shade and organic fall, pools you service every other week rather than weekly, a pool sitting between owners with nobody watching it, and the week after you have recovered a bloom, when the chlorine demand is still settling and a relapse is most likely. Everywhere else, weekly algaecide is buying insurance against a risk your chlorine is already handling.
The way to know which of your pools are actually in that group is history rather than memory. If you log readings and notes at every visit, you can see which pools keep coming back green instead of dosing the whole route because three pools scared you in July. That same record is the honest test of whether the product is working at all: a preventative that is earning its money shows up as fewer repeat algae calls on the same addresses, and if the pattern does not move, you are paying for a habit. It is the same evidence worth demanding before any add-on goes on the truck, which is how to decide whether phosphate remover is worth buying as well.
FAQ
Frequently asked questions
Do I need algaecide if my chlorine is always in range?
Usually not. A pool that holds free chlorine in the right proportion to its cyanuric acid, gets brushed, and turns its water over properly will not grow algae, and adding a weekly algaecide to that pool is buying insurance against a risk you have already handled. The honest exceptions are pools where chlorine predictably slips between your visits: heavy shade with organic fall, an every-other-week schedule in summer, a vacant property, or the week after you have cleared a bloom. On a typical residential route that is a handful of pools, not the whole book. If you find yourself dosing everything weekly, the money is going to habit rather than risk, and the same spend on stabilizer management or a longer pump run time would prevent more algae than the bottle does.
Why did my pool foam after I added algaecide?
You almost certainly used a quaternary ammonium product, and either overdosed it or added it to a pool with an aerated return. Quats are surfactants, which is part of how they work, and that same property whips into foam when the water is agitated - a spa jet, a waterfall feature, or a return aimed at the surface will do it. Foam is cosmetic rather than dangerous, but it looks like a problem to the customer and it does not clear quickly. Run the filter, keep the water agitation down, and it typically settles over a day or two; a small amount of anti-foam clears it faster. The lasting fix is to stop using quats on featured pools and switch to polyquat, which does not foam at any sane dose.
Should I add algaecide before or after shocking?
After, and not on the same visit if you can avoid it. A shock-level chlorine dose will oxidize most algaecides straight out of the water, so anything you add during or immediately after a shock is largely wasted money. Let the free chlorine fall back toward its normal range first, which usually takes a day or two depending on sun and stabilizer, then add the preventative to water that is already clear and sanitized. That sequence matches what the product is for: it holds the line after a treatment rather than assisting it. The one product that is deliberately added before shocking is sodium bromide for black algae, because the shock is what converts it to active bromine - that is a specific sequence, not a general rule.
Is polyquat worth the extra cost over a cheaper algaecide?
On a customer's pool, yes, because the price gap is smaller than it looks per dose. A quart of polyquat at $25-$40 against a quat at $10-$20 sounds like double, but a maintenance dose is a few ounces per 10,000 gallons, so the real difference on one pool is a couple of dollars a month. Set that against a foaming complaint or a stained wall and the cheap bottle stops being cheap. The place a quat still makes sense is a pool you see every week and watch closely, where a weak preventative is backed up by your own eyes. On anything you visit every other week, or anything with a water feature, the extra couple of dollars buys you out of the two failure modes that actually generate callbacks.
Does algaecide work on black algae?
Only as part of a sequence, and only after the mechanical work is done. Black algae grows a protective layer and roots into plaster, which is why a colony shrugs off both ordinary chlorine and any algaecide added on top of it. The order that works is to brush the heads open with a stainless or wire brush, shock the pool hard, and then use a copper-based product or sodium bromide as the follow-up that reaches what you have exposed. Skipping the brushing makes the whole dose pointless, no matter which bottle you buy. Expect to repeat it: black algae routinely takes several rounds over two to three weeks, and a pool that has had it once should be watched on the same spots for the rest of the season.
Can I charge a customer for algaecide as a separate line item?
Yes, and you generally should when it is a treatment rather than routine prevention. Most service agreements fold ordinary maintenance chemicals into the monthly rate, and customers accept that, but an algae recovery consumes shock, acid, and often an algaecide well beyond a normal month - that is a specific job on a specific date and it itemizes cleanly on the invoice. Put the product and the visit on their own lines with the date and what the water was doing, so the charge reads as work rather than a markup. Where operators get into trouble is quietly billing a weekly algaecide that the customer never agreed to and cannot see the benefit of. If it is prevention you have chosen for your own convenience, it belongs in your base rate; if it is recovery, bill it and show the readings behind it.
