Chlorine tablets (Trichloro–s–
A chlorine tab has two side effects that most pool owners don’t realize:
- It has a pH of 2.9 but more importantly, it lowers Total Alkalinity (TA).
- It is 52% cyanuric acid (aka conditioner or stabilizer) by weight and each one increases the cyanuric acid level (CYA) in your pool water.
So, chlorine tabs lower pH and Total Alkalinity and increase cyanuric acid levels. If used in moderation they work wonderfully but when it’s the lone source of chlorine your pool water eventually becomes acidic (low pH and TA) and the CYA level gets so high it becomes unmanageable and the only way to correct high cyanuric levels in the pool will be to dilute, or drain water and re-fill with fresh water. Draining pools in general is a bad idea for several reasons but was especially wasteful (and avoidable!) during California’s great drought (December 2011 to March 2017).
The ideal range for pH in a gunite swimming pool is 7.4 – 7.6. The ideal range for TA in a gunite swimming pool is 80-120 ppm. While pH naturally rises in gunite pools, using tablets lowers the pH. This is an added benefit of Trichlor tabs IF used in moderation. Overuse of tabs lowers the pH down below 7.2 and the water becomes acidic, affecting plaster surfaces, pool equipment and bather skin and eyes. Most importantly, tabs dramatically affect TA by driving down levels as low as 30 ppm. When the TA gets that low the plaster weakens and overtime softens and eventually erodes (have you ever found white sand in your Polaris bag? Its the aggregate coming out of the plaster. This is a tell-tale sign that the alkalinity has been low for years).
The ideal range for CYA in an outdoor swimming pool is 30-50 ppm (indoor pools don’t require CYA= no sunlight). Levels above 100 ppm adversely affect chlorine’s ability to disinfect. The higher the CYA level the higher quantity of chlorine required to disinfect effectively. This is one reason why you’ll add crazy amounts of chlorine to a pool yet it remains cloudy or green- the CYA is so high that it’s preventing the chlorine from killing the bad stuff. When this happens the CYA must be reduced and the only effective way to do so is draining and refilling with our valuable and critical California water supply 🙁
In contrast, I/we use liquid chlorine (sodium hypochlorite aka bleach) year around. We only use tablets in the summer as a chlorine time release as they dissolve throughout the week. UV rays of the sun break down chlorine and the tabs time release helps hold a chlorine residual in the pool until we return the following week for our next service visit (we add sodium bicarbonate to raise TA to appropriate levels after being offset by tab use).
Here’s the rub: tabs are easy to transport and store, they’re lightweight and have a great shelf-life. Bleach is a challenge to transport and spills ruin personal property. Its heavy and it its potency decreases daily. Still, for all its trouble, liquid chlorine has less side effects (no built in CYA, doesn’t lower TA) and makes the pool easier to balance and that in turn helps equipment and plaster to last longer (the candid goal of water chemistry!) We rarely have to drain a pool and when an algaecide is occasionally needed it isn’t rendered ineffective by high CYA levels.
So, are chlorine tablets bad for your pool? NO, if used sparingly, but, if they are your only source of sanitizer in your pool you will eventually develop problem$, its only a matter of time.