What is the difference between algaecide and shock?
The difference is what each one does to algae. Shock kills it; algaecide prevents it or weakens it so chlorine can kill it. They work at different points in the fight, which is why swapping one for the other does not work.
Shock is not a single product, it is a job: raising free chlorine well above its normal level to kill living algae and oxidize the waste feeding it. You can shock with cal-hypo, dichlor, or liquid chlorine. One catch matters here, though: non-chlorine shock (potassium monopersulfate) oxidizes the water and clears chloramines, but it does not kill algae, so for a green pool you specifically need a chlorine shock. Size the amount for your pool with the pool shock calculator, and the how to shock a pool guide covers the full routine.
Algaecide is a chemical that attacks algae cells directly, usually a poly-quat (PolyQuat 60) polymer that ruptures the cell wall, a copper or mineral compound that poisons the cell, or sodium bromide used to knock down stubborn mustard algae. On its own it will not clear a bloom that has already taken hold, because chlorine, not algaecide, is what does the heavy killing. Its real value is prevention between chlorine doses and helping against the strains chlorine has trouble holding down. Our best pool algaecide guide breaks down which type to buy.
Which one actually kills algae, shock or algaecide?
Shock kills algae; algaecide does not, at least not on its own. If the water is green or you can see algae on the walls, shock is the answer, every time.
Chlorine is the sanitizer that oxidizes and destroys algae, and shocking simply means driving free chlorine high enough and holding it there until the algae is dead and the water clears. The proven method for a green pool is to raise chlorine to shock level and keep it there, brushing daily and running the pump around the clock, until the water turns from green to cloudy gray to clear. That process is laid out step by step in the how to get rid of pool algae guide. Algaecide can help at the margins, but it cannot replace that chlorine.
Where algaecide does pull its weight is on the strains chlorine struggles to finish. Mustard (yellow) algae clings to walls and comes back from tiny survivors, so a sodium-bromide or poly-quat treatment alongside a hard shock helps clear it, as covered in the mustard algae guide. Black algae roots into plaster behind a protective cap and is the toughest of all; you break the cap, brush hard, and hold high chlorine, with algaecide as a supporting player, per the black algae guide. For ordinary green algae, though, chlorine does the work and algaecide is optional.
Should you shock or add algaecide first?
Shock first, then algaecide, and never both at the same time. A heavy dose of chlorine breaks down most algaecides before they can do anything, so adding them together wastes the algaecide and can leave a foamy mess.
The order that works is this: shock the pool, brush, and run the pump to kill the bloom and clear the water. Then wait for free chlorine to fall back to the normal 1 to 3 ppm range, usually about a day, before you add algaecide as a preventer. That way the chlorine does its killing without chewing up the algaecide, and the algaecide goes in when it can actually stick around and prevent regrowth. The how long after shocking can you swim guide explains how to know when chlorine has dropped back down.
This trips people up because pool stores sell shock and algaecide together as an opening or weekly bundle, which makes it look like they go in at once. They do not. Add the shock, let it work and dissipate, then add the algaecide. If you are only doing weekly prevention on a clear pool, a maintenance dose of algaecide after your normal chlorination is fine; the timing rule matters most when you are shocking hard for an active problem. Size any chlorine dose with the pool chlorine calculator.
Do you even need algaecide if you shock regularly?
Not usually. A pool with steady free chlorine and balanced water rarely gets algae, so algaecide is optional insurance rather than a must-have. Chlorine that is kept in range is the real algae prevention.
Most green pools are not a chlorine-product failure, they are a chlorine-level failure: the free chlorine got too low for too long, often because cyanuric acid was too high and throttling the chlorine, or the pump was not running enough. Fix those and algae has a hard time getting started. That is why the honest answer is to nail your chemistry first and treat algaecide as a backstop. Check your targets against the ideal pool chemistry levels.
There are a few times algaecide genuinely earns its keep: as a weekly preventer in hot, sunny weather when chlorine burns off fast; when you close the pool for winter and chlorine will sit unmanaged for months; and against mustard and black algae, which resist chlorine more than green does. For those cases a non-metallic poly-quat is the safe default, because copper algaecides can stain a pool and add metal to the water. Pick the right type in the best pool algaecide guide, and match your shock to the job in the best pool shock guide.
Algaecide wins on
- +Prevents algae from getting started, so a clear pool tends to stay clear.
- +The right backup against mustard and black algae, which resist plain chlorine.
- +Cheap insurance for low-chlorine stretches like winter closing.
Shock wins on
- +Actually kills algae; the only tool that clears an active green pool.
- +Also destroys chloramines and organic waste, restoring the water in one step.
- +Uses chlorine you already keep on hand, with the dose easy to size for your pool.
The verdict
For killing algae you have right now, shock wins, every time, because chlorine is what kills algae and algaecide is not. If the water is green, raise free chlorine to shock level and hold it there while you brush and run the pump, using a chlorine shock (cal-hypo, dichlor, or liquid), not a non-chlorine shock, which oxidizes but does not kill. Algaecide is a preventer and a backup, not a substitute: add it after you shock, once free chlorine has fallen back to the normal 1 to 3 ppm range, and never at the same time, because a heavy chlorine dose breaks most algaecides down before they can work. Where algaecide earns its place is weekly prevention in hot weather, winter closing when chlorine sits unmanaged, and the stubborn mustard and black strains chlorine struggles with, in which case reach for a non-metallic poly-quat so you do not stain the pool. If you can keep only one on hand, keep shock, and if you keep both, use them in order. Size your dose with the pool shock calculator, and follow the full green-pool routine in the how to get rid of pool algae guide.
Related: Pool shock calculator, Best pool algaecide, How to get rid of pool algae.
Frequently asked questions
Should you shock or add algaecide first?
Shock first, then algaecide, and never both at once. Shock the pool to kill the algae and clear the water, then wait until free chlorine falls back to the normal 1 to 3 ppm range, usually about a day, before adding algaecide as a preventer. A heavy dose of chlorine breaks down most algaecides, so adding them together wastes the algaecide and can create foam. The how long after shocking can you swim guide explains how to tell when chlorine has dropped back down.
Does algaecide work better than shock for killing algae?
No. Shock is far better at killing algae because chlorine is what actually kills it, while algaecide mostly prevents algae or weakens it so chlorine can finish the job. For an active green pool, shocking and holding a high chlorine level is the method that works, laid out in the how to get rid of pool algae guide. Algaecide helps most against mustard and black algae and as ongoing prevention, not as a stand-alone cure for a bloom.
Can I add algaecide and shock at the same time?
No. A heavy dose of chlorine degrades most algaecides before they can do their job, so adding them together wastes the algaecide and often leaves the water foamy. Add the shock first, let it kill the algae and dissipate, then add algaecide once free chlorine is back near 1 to 3 ppm. Pool stores sell them bundled for openings, but that does not mean they go in at the same moment; it means you use both, in order.
When should you put algaecide in a pool?
Add algaecide when the water is clear and free chlorine is at a normal level, not during a shock. The best times are a weekly preventive dose in hot, sunny weather when chlorine burns off fast, right before you close the pool for winter, and as a backup when fighting mustard or black algae. A non-metallic poly-quat is the safe default because copper algaecides can stain the pool. See which type to buy in the best pool algaecide guide.
Is it bad to add too much algaecide to a pool?
Yes. Overdosing a poly-quat algaecide is the most common cause of a foamy pool, since the polymer acts like a surfactant and whips up bubbles at the returns. Too much copper-based algaecide is worse: it can stain the pool and turn light hair or plaster green as the copper deposits. Follow the label dose for your pool volume, add it with the pump running, and never pour in extra to try to speed up killing an existing bloom, because that is a job for chlorine, not algaecide.