Liquid chlorine (sodium hypochlorite) and chlorine tablets (trichlor) are the two most common pool sanitizer products. They have the same end goal but very different chemistry and side effects.
Key Facts
- Liquid chlorine (sodium hypochlorite) is typically 10–12.5% strength and raises pH slightly.
- Trichlor tablets are 90% available chlorine but also add cyanuric acid with every dose.
- CYA buildup from tablets can reach 80–100 ppm in a single season without a partial drain.
- Liquid chlorine is the better choice when CYA is already at or above the target range.
Liquid Chlorine (Sodium Hypochlorite)
Liquid chlorine is sold as a 10–12.5% sodium hypochlorite solution. When added to pool water, it dissociates into sodium and hypochlorite ions. It raises pH slightly (typically by 0.1–0.2 per dose for average pools) and does not add cyanuric acid. Liquid chlorine is the standard choice for regular sanitisation and for shock treatments because it takes effect immediately and has predictable, simple chemistry. Its main drawback is that it degrades over time in storage — use it within 60 days of purchase and store it away from heat and light.
Chlorine Tablets (Trichlor)
Trichlor (trichloroisocyanuric acid) tablets are a convenient, slow-dissolving form of chlorine. Each tablet contains approximately 90% available chlorine but also contains cyanuric acid as a stabiliser. Every pound of trichlor added to a pool raises CYA by approximately 0.6 ppm for every 10,000 gallons. Used as the primary sanitiser through a full season, tablets can drive CYA above 80–100 ppm — the level at which chlorine effectiveness is significantly reduced. Tablets also lower pH with each dose, so pH increases are typically needed when using tablets as the primary sanitiser.
Which to Choose
If your CYA is already at or above 50 ppm, switch to liquid chlorine for regular sanitisation. If your CYA is low or you are starting a new season, tablets are acceptable for daily maintenance dosing and are very convenient for slow-dissolving feeders and floaters. For shock treatments, always use liquid chlorine, calcium hypochlorite, or non-stabilised shock — never use trichlor or dichlor tablets as shock because they add unneeded CYA at the high doses required for shocking.
Examples
A 15,000-gallon pool using two 3-inch trichlor tablets per week adds approximately 0.9 lbs of product per week. Over a 20-week season, that is 18 lbs of trichlor. Each pound adds 0.6 ppm CYA per 10,000 gallons, so 18 lbs adds approximately 10.8 x (15,000/10,000) = 16 ppm CYA per week — no, per season. Starting from 30 ppm in spring, CYA reaches roughly 48 ppm by end of season. That is within range, but in a second season it would reach 66 ppm. Testing CYA monthly and doing a partial drain when it exceeds 60 ppm prevents buildup.
Common Mistakes to Avoid
- Using trichlor tablets as shock and accumulating CYA to levels that make the pool unsanitisable.
- Not testing CYA monthly when tablets are the primary sanitiser — CYA accumulates invisibly.
- Storing liquid chlorine for more than 60 days, by which time its concentration has degraded significantly.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference