Algaecide + Shock: Safe Combo?

Sanitizers, clarifiers, purifiers, conditioners, stabilizers, algaecides, bromides, chlorine, salt, acid, flocculant – who said looking after the water in your pool was simple? Well, it is, once you’ve worked out how to maintain the balance of chemicals that keep the water sparkling and healthy. Two questions often asked are whether an algaecide is necessary and whether algaecide and shock can be used together.

Algaecide and shock are both formulated to prevent algae and bacteria from affecting the healthy state of pool water, but because they don’t work in combination, they cannot be used together. In fact, they negate each other’s effectiveness, and combining them could cause a dangerous chemical reaction.

You don’t need to be a qualified chemist to understand the various functions of all the additives I’ve mentioned, but it helps! I will concentrate in this article on two of the most important, algaecides and shock, and look at what they are, what to expect from them, how they differ in function, and how to use them most effectively.

Algaecide And Shock Together – Why Not, And What’s Best?

To understand how best to use these two additives in your pool, I’ll briefly explain each product and how they differ in composition and purpose.

What Is An Algaecide?

Despite their name, the primary role of algaecides is not only to kill algae but rather to act as a back-up, preventing them from growing and multiplying after they have been eliminated by other chemicals in the water. Algaecides are available in three forms:

  • Quat (short for “Quaternary Ammonium”) algaecides, which are the cheapest form, act as a detergent to wash algae out of the water. Adding too much can cause foaming, and they tend to have a strong smell.
  • Polymer (Polyquat) algaecides are non-metallic, non-foaming, and non-staining. They are twice as effective as quat products and are formulated using ammonia compounds. They have a positive charge and attach to the negatively charged algae cells, covering their surfaces and smothering them.
  • Metallic Pool Algaecides are the most powerful of the three but are also fairly expensive. They use mainly copper, which combines with an amino acid to form organocopper ions that reduce the risk of staining. These are positively charged and enter the algae cells and poison them from within. They have the advantage of being effective against black and mustard algae as well as green.

What Is Shock?

Shock is the term given to the additive that gives a super-dose of active chlorine to pool water, which kills off bacteria, chloramines, dead skin cells, body oils, and other contaminants, including all types of algae.

It’s available in three forms:

  • Calcium Hypochlorite (known as cal-hypo) is the most cost-effective way of super-chlorinating your pool.
  • Sodium di-chlor (usually referred to as di-chlor), is stabilized so the chlorine dissipates more slowly. The benefit of this type of shock is that it doesn’t add calcium to the water – calcium can build up over time and block the filtration system if not removed.
  • Potassium Monopersulfate: This doesn’t increase chlorine levels but makes the existing chlorine more effective. It’s unaffected by sunlight, and the pool is safe for swimmers after only 15-20 minutes.

What Happens When You Mix Shock And Algaecide?

As you can see from the description I’ve given, algaecides and shock are very different, although they both are used to remove contaminants from the water. Mixing them might not always be dangerous, but it may have unwanted results:

  1. It may cause both to become ineffective, as the various chemicals may react with each other instead of with the contaminants in the pool.
  2. Pool shock is highly reactive and, when mixed with other chemicals, might release toxic gases and, in extreme cases, might explode if you mix the two ingredients before adding them to the pool.

Algaecide Or Shock – Which One First?

Okay, it’s definitely not a good idea to mix the two, despite some opinions to the contrary – so what’s the right way to use an algaecide and a shock so that they’re both effective?

Adding Shock First

Because a high dose of chlorine is the best way to kill algae and other bacteria, a shock treatment of your pool is the first step in turning the pool from green to blue.

  1. Measure your pool’s chlorine level, pH, and alkalinity to determine how much shock you need. If necessary, get the balance right before you go ahead, and run the pump for an hour or two to circulate the added chemicals.
  2. If you’re using a granular shock, dissolve the correct amount (following the manufacturer’s instructions) in a bucket of water.
  3. Run the pump and pour the dissolved shock solution slowly into the pool, starting at the deep end and emptying the contents of the bucket slowly around the perimeter. I also recommend pouring the shock into the water at the return nozzle to ensure it’s well-circulated.
  4. Especially if you’re using unstabilized shock, pour it into the pool after the sun has lost its heat, in late afternoon or evening, so that the chlorine dissipates more slowly, and then run the pump for at least two to three hours before switching off for the night.

Adding Algaecide Later

Because algaecides are designed to support chlorine in its role of algae-killer, it’s always better to add it after shock treatment and only after the chlorine level has dropped back down to approximately 2-4ppm (parts per million). At that stage, the shock will have done its work, and the algaecide will take effect.

The only exception to this rule (there’s always one!) is when there’s a rain- or thunderstorm brewing before your next scheduled shock treatment. Both rain and lightning affect the pool by adding organic contaminants (particularly nitrogen) to the water. An algaecide added before the storm can limit the algae bloom that often results and will control the growth before you shock the pool again.

Can I Use Shock Instead Of Algaecide?

Chlorine is the best way of killing green algae, but is not effective against black or mustard varieties, and this is where a good algaecide is needed. Because chlorine levels drop quickly, opportunities arise all the time for algae to bloom, and, again, the algaecide will prevent this. So, while both additives are used to control algae, they work in conjunction with, rather than an alternative to, each other.


Algaecides and shock are chemically very different from each other, and for this reason, they react with each other when used together. The result is that neither of them is effective in controlling the growth of algae and might even cause a harmful chemical reaction. If the shock is used 24-48 hours before the algaecide, they both work exceptionally well in keeping the pool clear, healthy, and contaminant-free.