Algaecide Or Shock First

Algaecide Or Shock - Which Do You Add First?

Algaecide and shock treatments are both super effective at removing algae infestations from your pool. You’ve already done your research and learned that algaecide is best used in conjunction with a shock or chlorine treatment, but which should you add first?

In this article, we’ll talk about whether you should use chlorine shock or algaecide first and why this distinction is important.

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Which Do You Use First?

Shock treatments, including chlorine based ones and non-chlorine types, can have chemical interactions with algaecide products that render them ineffective or even harmful.

Because of this, these products cannot be used at the same time, or while there is a large concentration of one or the other present in your pool’s water.

It is recommended that you perform a chlorine shock treatment first, before adding your algaecide.

However, it is important that you let your chlorine levels return to a near normal amount of 2-4 parts per million (ppm), or as otherwise indicated on your algaecide’s instructions before adding any additional product.

Chlorine levels tend to take around 48 hours to return to normal after a shock treatment, but this effect can vary depending on many pool conditions, as well as the amount used and concentration of your shock product.

Always test your pool water’s chlorine levels before adding your algaecide.

Why It’s Important

Algaecides and shock products contain many different active chemicals, minerals, and even metal compounds that are designed to kill off living organisms in your water as they change forms.

When these products come in contact with each other, they tend to react with the chemicals found in the other product, which renders these once beneficial compounds completely ineffective.

In cases where there are some active compounds remaining, you will not be getting near what you paid for in terms of product effectiveness.

Some shock products and algaecides may also contain chemicals that can become dangerous to handlers or swimmers when combined. To avoid harmful chemical interactions, always follow safety instructions provided.


When it comes to whether you should use algaecide or shock first in your pool, there is a clear answer.

Shock treatments should be used before algaecides to create as inhospitable an environment for microorganism growth as possible, followed by algaecide to kill the existing algae in your pool.

The free chlorine in the shock treatment will also dissipate much faster, minimising the chance of your products becoming ineffective when they interact chemically.

Note: If you need help calculating an appropriate shock or algaecide dose, use this chlorine shock dose calculator or algaecide calculator.

Do you have any questions about whether you should use algaecide or chlorine first in your pool? Leave us a comment down below!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.