Pool algaecide: when it helps and how to use it
Algaecide is a backup to chlorine, not a replacement. A pool that keeps free chlorine at the right level for its stabilizer doesn't grow algae, with or without algaecide. Where algaecide helps is in covering gaps: a week away, a closed pool, or a pool that keeps getting the same stubborn algae.
Main types
| Type | How it works | Watch out for |
|---|---|---|
| Quaternary ammonium ("quat") | A surfactant that disrupts algae cells | Can foam, especially when overdosed. Uses up some chlorine. |
| Polyquat (often labeled "60%" or "non-foaming") | A polymer form of quat | Less likely to foam. Usually costs more. |
| Copper-based | Copper ions are toxic to algae | Copper can stain plaster and tint hair or water, especially with high pH or low stabilizer. Avoid with metal-sensitive surfaces. |
Product strengths and doses vary a lot between brands. Follow the label for your product.
When algaecide is worth using
- Before closing for winter, or a long period with no one testing or adding chlorine.
- Before a vacation, as a backstop if chlorine runs low.
- After clearing a stubborn algae, such as mustard or black algae, to discourage it from returning.
- Pools with low circulation spots where algae keeps starting, while you fix the flow.
When you don't need it
If free chlorine stays at or above about 7.5% of your stabilizer level every day, algae can't establish. In that case, algaecide is an extra cost with no real benefit. If algae keeps appearing, the fix is almost always more consistent chlorine: see the chlorine and stabilizer guide.
Example: a pool with 40 ppm stabilizer needs at least 3 ppm free chlorine. If it routinely drops to 1 ppm by evening, algaecide might slow algae, but bringing chlorine up to 3–5 ppm solves it. The chlorine calculator gives the dose.
Algaecide doesn't clear a green pool
Once algae is growing, the fix is to shock to about 40% of your stabilizer level and hold it until the water is clear and passes an overnight chlorine loss test. See how to shock a pool. Adding algaecide to a green pool mostly adds cost, and quat algaecides can foam in water full of debris.
How to use it
- Make sure chlorine and pH are in range. Algaecide works alongside chlorine, not instead of it.
- Measure the dose from the label for your pool's volume. If you don't know it, use the pool volume calculator.
- Add it around the edge of the pool with the pump running, or as the label directs.
- Don't add it at the same moment as a large chlorine dose, unless the label says that's fine. Many labels suggest waiting until chlorine is back in its normal range.
- Keep the pump running so it mixes.
Frequently asked questions
Why is my pool foamy after algaecide?
Quat algaecides act like a mild detergent. Overdosing, or adding to water that already has some, causes foam. See foamy pool water.
Can I swim after adding algaecide?
Follow the product label. Many allow swimming once it has circulated.
Does algaecide prevent black algae?
It can help slow it down after treatment, but black algae is best prevented by brushing and steady chlorine. See pool algae types.