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Clear algae in this order: test and drop pH to about 7.2, shock hard with unstabilized chlorine at dusk, brush every surface, run the pump continuously for 24 hours, then vacuum out the dead algae. Green algae usually clears in two to four days; mustard takes about a week; black algae takes weeks. Algaecide prevents blooms but rarely rescues one. Never mix chemicals with each other, never combine chlorine with acid, and always add product to water rather than water to product.
An algae bloom is not a hygiene failure. It is a chlorine deficit that lasted long enough for spores to take hold, and warm water is unforgiving — a pool that looked slightly dull on Friday can be fully green by Sunday. Which means the response time matters far more than the product you pick.
The treatment sequence
- Test everything first. Free chlorine, pH, total alkalinity and — critically — cyanuric acid. If stabilizer is above roughly 100 ppm, no realistic amount of chlorine will work and you need to dilute the water before treating.
- Bring pH down to about 7.2. Chlorine is far more effective at the low end of the range, because more of it exists as the active hypochlorous acid form. Shocking into pH 8.0 water is why some treatments seem to accomplish nothing.
- Clean the filter and check circulation. A clogged cartridge or a channelled sand bed cannot remove what you are about to kill. Confirm the return jets actually stir the whole pool.
- Shock hard, at dusk. A visible bloom needs the algae dose on the label, typically two to four times the routine rate, not a maintenance dose. Dusk matters because unstabilized chlorine in full sun loses a large share of its strength within a couple of hours.
- Brush every surface — walls, floor, steps, ladder pockets, behind the light niche, the shaded side. Brushing breaks the protective film over an algae colony and exposes it to chlorine. This step does as much work as the shock.
- Run the pump continuously for 24 hours and hold chlorine at shock level. The water going from green to cloudy gray or milky blue is the visual confirmation the algae is dead.
- Vacuum out the remains. Let the dead algae settle overnight with the pump off, then vacuum to waste if your filter has a multiport valve, so the debris leaves the system rather than clogging the filter.
- Repeat if chlorine crashes. Retest the following morning; a heavy bloom often consumes the entire first shock. A second dose is normal.
- Rebalance and resume. Once the water is clear, bring pH back to 7.4 to 7.6, restore free chlorine to 2 to 4 ppm, and only swim once chlorine is back in that range.
Chemical safety, without the euphemisms
More pool owners are hurt during an algae treatment than at any other point in the season, because this is when they handle concentrated product in a hurry.
- Never mix any two pool chemicals — not in a bucket, not in a scoop, not in the skimmer. One product, pump running, wait the label interval, then the next.
- Never combine a chlorine product with an acid. The reaction releases chlorine gas.
- Never mix two chlorine types. Cal-hypo granules contacting trichlor residue can ignite. Separate scoops, separate storage.
- Always add chemical to water, never water to chemical.
- Dose downwind, in eye protection and gloves, spreading granular product across the deep end rather than dumping a pile onto a vinyl liner, which bleaches it.
- Keep everyone out until chlorine has fallen back to 1 to 4 ppm.
Best for clearing a green pool fast in a plaster or vinyl pool with low calcium
- 68 percent available chlorine, unstabilized so it adds no cyanuric acid
- Pre-measured 1 lb bags — no scooping from an open pail
- Roughly 1 lb treats 10,000 gallons at a maintenance dose
- Raises calcium hardness slightly as it works
Why buy it: unstabilized cal-hypo delivers the heavy chlorine an algae bloom needs without pushing cyanuric acid higher, which is what stalls repeat treatments.
Best for preventing regrowth after the shock has done the killing
- Non-foaming formula, so it will not froth in a spa or high-flow return
- Works against green, yellow and black algae
- Dosed weekly as prevention or heavier as a treatment
- 40 oz bottle covers a season on an average residential pool
Why buy it: algaecide is a poor rescue tool and an excellent insurance policy — used weekly it stops the next bloom starting during the week your chlorine dips.
We never buy or physically handle what we rank — our verdicts come from aggregated owner ratings and expert consensus, described on how we rank. The full fields are in the best pool shock and the best pool algaecides.
The three algae types behave very differently
| Type | What it looks like | Difficulty | Approach |
|---|---|---|---|
| Green | Whole pool turns green; slippery film on walls | Easiest | Shock, brush, filter, vacuum. Clear in 2 to 4 days |
| Yellow or mustard | Dusty yellow-brown patches on shaded walls; looks like sand | Moderate | Brushes off easily but returns. Higher shock level, and every item that entered the pool must be sanitized too |
| Black | Dark raised spots rooted into plaster and grout | Hardest | Stainless brush to break the cap, spot-treat, repeated shocking. Weeks, not days |
The distinction that catches people out is that mustard and black algae both survive outside the water. They live on swimsuits, floats, nets, brushes and vacuum heads, and reintroduce themselves days after a treatment that looked completely successful. If you are fighting a recurring bloom, run the pool gear and the swimwear through chlorinated water at the same time as the pool treatment, or you will simply reinfect the water on the first swim.
Black algae in particular has a protective cap that ordinary brushing will not break. It needs a stainless brush on plaster — never on vinyl, which it will destroy — and genuine force, followed by spot treatment directly onto the exposed head.
Why a treatment stalls
Owners who report that shocking “did nothing” almost always have one of five specific problems, and none of them is solved by adding more chlorine.
| Symptom | Cause | What to do |
|---|---|---|
| Chlorine tests high, pool stays green | Cyanuric acid above about 100 ppm has locked the chlorine up | Partial drain and refill; switch to unstabilized chlorine |
| Shock dose disappears within hours | Heavy organic load consuming it, or dosed in full sun | Redose at dusk and hold the level for a full 24 hours |
| Green clears, returns within days | Mustard or black algae surviving on gear and swimwear | Sanitize floats, nets, brushes, vacuum heads and swimsuits at the same time |
| One corner stays green | Circulation dead spot, not a chemistry failure | Aim a return jet into it; brush that area weekly |
| Water goes gray then hazy again | Dead algae passing back through a sand filter | Vacuum to waste, add a clarifier, run the pump continuously |
The first row is the one worth checking before anything else, because it makes every other effort futile. Stabilizer never leaves the water on its own — it is not consumed and it does not evaporate — so a pool run on trichlor tablets accumulates it season after season until the chlorine reading becomes meaningless. If a pool has fought algae repeatedly over several summers despite good chlorine numbers, test cyanuric acid first.
Stopping the next one
Prevention is entirely a matter of never letting free chlorine reach zero, and the numbers below are what make that achievable.
| Measure | Target | Why it prevents algae |
|---|---|---|
| Free chlorine | 2 to 4 ppm at all times | The actual sanitizer. A bloom means this hit zero |
| pH | 7.4 to 7.6 | Above 7.8 much of your chlorine is chemically inactive |
| Total alkalinity | 80 to 120 ppm | Keeps pH from swinging after rain or heavy use |
| Cyanuric acid | 30 to 50 ppm | Protects chlorine from sun; above 100 ppm it disables it |
| Pump run time | Full daily turnover | Dead spots grow algae even when the rest of the pool is fine |
Add weekly brushing of the steps, ladder pockets and the shaded wall, a weekly preventive algaecide dose if your pool has a history, and a chlorine top-up after any heavy rain. If blooms keep starting in the same corner every time, that is not chemistry — it is a circulation dead spot, and aiming a return jet at it usually solves what a year of shock did not.
Where to go next
Get the underlying numbers right first: pool water chemistry covers all five and the order to adjust them. If the water is dead but still hazy, cloudy pool water is the filtration side of the same problem. For the removal step see how to vacuum algae out of a pool, and if chlorine vanishes between tests, why my pool keeps losing chlorine. Everything else sits under the pools and spas hub.