Algaecide for your Pool: How much do you need to add, How to use it and How does it work? - Sutro, Inc

Algaecide for your Pool: How much do you need to add, How to use it and How does it work?

Algaecide can help prevent algae in pools, but it may shock (more on that word later) you to learn that it’s not really needed for everyday pool care.

Algaecide is thought of as the secret potion to fix all of our algae problems but it is really more of a preventative than treatment and should be used only when it’s appropriate.  

Let’s see how it works.

What is algaecide and how does it work?

Algaecide is any substance that is poisonous to algae and it can be found in liquid variants and mineral form.

It’s generally agreed that algaecides work by disrupting cellular functions in algae and compromising their walls and reproductive capabilities.

Keep the above in mind because by compromising the walls you allow chlorine to be more effective at penetrating and killing algae.

This also means that if your chlorine level is low it can still be effective with the help of algaecide (vacation times for example).

What does algaecide do for a pool and when should it be used?

Algaecide helps you stop getting rid of algae growth and helps your sanitizer be more effective at killing the algae.  

This can be particularly beneficial when closing a pool for the winter, for example. By adding algaecide as part of your winterizing routine you can help your sanitizer over the winter and during the spring when temperatures may fluctuate and be warmer for several days before you are ready to actually open your pool.

Another situation when algaecide can be beneficial is when you are on vacation and your sanitizer levels may drop below-recommended levels.

Similarly, if something happens to your pump or salt cell or other equipment that would disrupt circulation and sanitation then having algaecide in the water can also help short-term as an insurance policy against an algae bloom.

We are talking days here, not weeks so don’t drag your feet on repairs or you could end up having to pay more to recover from a green pool.

Following the same rationale, another example when algaecide can help is during an ascorbic acid stain treatment where you need to keep your chlorine low while the acid removes your stains.

Calculate how many gallons your pool has with our pool volume calculator.

What if my pool is green or cloudy, is algaecide enough?

The short answer is simply “no.”

Algaecide alone is not the solution for a green pool, and in fact algaecide isn’t even needed when treating a pool that’s green due to an algae outbreak. In some cases it can do more harm than good. The cure for an algae outbreak is sanitizer, and a lot of it so that you shock the water and destroy the algae.

We discuss more about the types of algaecides here, but be aware that if you use an algaecide that contains copper you can actually stain your pool or your hair and fingernails if the copper concentration is high enough when you shock your pool.

We also have a post on how to get rid of algae in your pool that can help you get rid of algae.

If left untreated, or if black algae is bad enough, then an acid wash is sometimes required in extreme cases. An acid wash will remove the alage, but it does so by removing the top layer of the plaster,so it’s best to keep algae away and if it is present then get rid of it before it gets out of control.

Should I use it for opening my pool?

Algaecide is not a requirement for opening your pool, and we would recommend against it unless you have some chlorine-resistant algae like black algae or mustard algae that is giving you problems.

Another reason for adding algaecide at startup would be if your cyanuric acid levels are too high due to the overuse of stabilized chlorine tabs. Having high cyanuric acid inhibits chlorine effectiveness and algaecide can help your chlorine be more effective until you get your cyanuric acid level under control. 

More importantly, for starting the season off right and maintaining good water quality you should make sure that you properly maintain your water chemistry, brush your pool regularly, and make sure you are filtering your water long enough to prevent algae from showing up and ruining your party. 

Algaecide for above ground or inground pool, is it the same?

Yes, the only difference is the amount you would use. 

Using algaecide in an above ground pool for winterizing is helpful because above ground pools contain less water and are more susceptible to temperature swings during the spring. When there may be a few days of warmer weather before opening the pool, the algae can begin to grow and then lay dormant waiting for a longer stretch of warm weather to really start wreaking havoc. 

What algaecide to buy and where?

Choosing an algaecide can be very confusing and unfortunately it depends on your situation. In order to choose the right algaecide, you should understand what type of algae you are fighting and then choose based on that. 

Here are a few tips that may help:

  • If you are using algaecide as a preventative measure then a broad spectrum algaecide is best. No matter what kind you choose, make sure your water is balanced before using it so that you can minimize the risk of staining and/or maximize its effectiveness. 
  • It’s also important to realize that if you use a copper based algaecide, and you use a chelating or metal out type additive, you will counteract the algaecide because the ions will be bound by the other product and they can’t attack the algae. 
  • Avoid copper-based algaecides and opt for polyquat or mineral systems when possible.Keep in mind that algaecides  will require a maintenance dose periodically so be sure to read the manufacturer label and follow their guidance for the bewt results 

Algaecides can be purchased from big box stores, hardware stores, and of course from pool supply stores. 

How much algaecide should I use?

Determining the amount of algaecide to use is pretty easy based on how many gallons of water your pool holds and what the manufacturer instructions are.

For example, here is the common doses for the Sani Marc brand “Summer Smiles ALG OUT ULTRA 40%

DOSAGES Pool less than 40,000 L Pools 40,000 L to 80,000 L
ROUTINE DOSAGE (weekly) 100 mL 200 mL


Do salt water pools use it too?

Salt water pools are the same as non-salt water pools with regards to algaecide. The salt content doesn’t matter when using algaecide.

What about winter?

Algaecide is helpful when winterizing a pool because it will last longer than chlorine, especially in temperatures of 50ºF and lower. In addition, if your pool is uncovered and exposed to the sunlight during the winter then your chlorine will disappear faster too. 

Final thoughts / Conclusion

So there you have it, algaecide isn’t necessary for keeping your water clear, but it can help in some cases when your sanitizer level may drop below-recommended levels. 

There are different types of algaecides to treat different types of algae so it’s important to consider what you want to use it for and then choose based on that. 

The best solution for using algaecide is to keep your sanitizer level in range and prevent the need for algaecide all together. 

Sutro pool monitoring system can help you by testing your water 3x/day and telling you if your sanitizer is low or if your pH and alkalinity are out of range and then telling you how to adjust your water so that you avoid algae


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