Shocking your pool is a necessary evil. At least, if you want to get rid of chloramines — a.k.a. combined chlorines, the irritating kind of chlorine. When your pool water gets extra dirty, adding chemicals to shock your pool and raise your chlorine level helps to kill off algae and bacteria.
There are 2 types of chlorine in your water. Here’s a quick rundown:
- Free chlorine is powerful enough to beat out whatever it comes into contact with: bacteria, algae, other chemicals — you name it. Think of free chlorine as the good guy.
- Combined chlorine is what happens when chlorine meets ammonia or nitrogen. Since it’s combined with another substance already, it’s too weak to fight off bacteria, which stinks. Literally.
What happens when you shock your pool
For traditional pool chemistry, you need to maintain a free chlorine reading around 3 to 5 Parts Per Million (ppm).
To get rid of gunky combined chlorine, you need to "shock" the pool to breakpoint chlorination, which happens when there’s ten times more free chlorine than combined chlorine. Basically, you’re blowing up the combined chlorine.
So, where does dichlor come in?
To understand how dichlor works, you need to understand how free chlorine interacts with other chemicals in your pool. In particular, cyanuric acid (CYA) levels are often referred to as "stabilizer" levels, because if the CYA is kept high enough, chlorine levels won't fluctuate as quickly.
Dichlor shock is just a form of chlorine (short for sodium dichloro-S-triazinetrione) designed for saltwater and spas. Most chlorinators use stabilized chlorine pucks, which are hardened tablets of either dichlor or trichlor. Trichlor is a solid compound with the highest possible chlorine content. Both release slowly and maintain the free chlorine and CYA levels in the pool simultaneously.
Either way, you’ll have to do manual readings and corrections to check for balance.
What dichlor gets right
Granular dichlor is sold to get around needing to maintain free chlorine and CYA separately. In commercial pools, free chlorine and CYA each get their own reading (like a system of checks and balances). By using dichlor, users can raise their free chlorine and CYA levels at once. With granular dichlor, you don’t even spend time mixing anything — it can just go straight into the water.
Where dichlor goes wrong
Remember how we said commercial pools maintain free chlorine and CYA separately? There’s a reason for that. Typically these pools use 67% calcium hydrochlorite to raise the chlorine and follow up with granular cyanuric acid to raise the CYA. If you shock your pool with dichlor instead, you send CYA levels skyrocketing — which isn’t a good long-term solution for your pool chemistry. Just because the product is packaged for ease of use doesn’t mean it’s the best solution.
The Indiana State Department of Health doesn't think you should shock with any stabilized chlorine products: (see the red text on page 4): https://www.in.gov/isdh/files/How_To_Shock_The_Pool.pdf
On the other hand, several great pool chemical brands sell dichlor-based shock products: https://www.cloroxpool.com/products/shock-xtrablue/