When combined chlorine (chloramine) builds up too much in your pool, it can start to prevent free chlorine from effectively doing its job, as well as causing skin and eye irritation.

To remove the chloramines from your pool water, you’ll need to “shock” your pool. This process involves adding 10x the amount of chlorine to your pool as there is chloramine, which breaks up the chloramine molecules, causing them to off-gas (oxidize).

It’s a good idea to do this once per week to prevent a strong buildup of chloramines, but shocking can also be used to fight an algae bloom or other contaminants entering the water, such as human waste. Yep, it happens.

In theory, you can shock your pool with any type of chlorine as long as you add enough of it, but calcium hypochlorite is particularly suited because, as mentioned above:

• It’s strong, which means you need less product to reach 10x the level of combined chlorine (or what’s known as “breakpoint chlorination”)
• It’s fast-acting because it’s already dissolved when it enters the water. It takes about around 8 hours to do its thing, and you’ll leave it to work overnight because…
• It’s unstabilized, which means the sun will burn it off quicker. In the case of shocking, you actually want this to happen because you only need short burst of chlorine before bringing the chlorine level back down to normal.

And because shock requires a less frequent application, the added calcium and slight increase in pH is much easier to manage, especially when the rest of your water chemistry is properly balanced.