Bob Lowry Answers the Trichlor Debate

Trichlor chlorine tablets are popular in pool maintenance because they are convenient, slow-dissolving, and easy to use in a floater or inline chlorinator. The problem is not that trichlor “doesn’t work” as a sanitizer, but that it changes water chemistry in ways that compound over time. As trichlor becomes the primary or only sanitizer, cyanuric acid (CYA) steadily rises, pH steadily falls, and the pool becomes harder to keep stable from week to week. For pool service pros and homeowners alike, the hidden cost is chasing readings and fighting algae even when test results seem normal.

The core chemistry issue is cyanuric acid buildup. Trichlor adds stabilizer every time it adds chlorine, so the more you rely on tablets to meet a pool’s daily chlorine demand, the more CYA accumulates. As CYA climbs, it slows chlorine’s killing power, which means the same free chlorine reading becomes less effective at preventing algae and keeping water safe. A practical way to think about this is the chlorine to cyanuric acid ratio. Many industry recommendations center on keeping free chlorine around 5% of the CYA level (a 20:1 CYA-to-chlorine relationship). If CYA is 50 ppm, about 2.5 ppm free chlorine can be fine, but at 100 ppm CYA you may need about 5 ppm free chlorine to maintain the same sanitizing strength.

That dynamic creates the “tablet trap”: rising CYA requires higher chlorine targets, and using more trichlor to reach those targets adds even more CYA. Over weeks and months, this can lead to a pool that looks like it has “enough chlorine” on a basic test, yet still grows algae or stays dull because the chlorine is bound up by stabilizer. High CYA also complicates testing and balancing because it affects total alkalinity interpretation. Many pool chemistry approaches require adjusting the alkalinity reading by accounting for a portion of CYA to estimate carbonate alkalinity, which is what really buffers pH. When numbers are misunderstood, techs can overcorrect and create bigger swings.

Trichlor also brings safety and handling risks that get overlooked in day-to-day pool service. It is very acidic (low pH), so it tends to drive pH and alkalinity downward, forcing frequent corrective dosing. On top of that, stored tablets can “gas off” when moisture gets into the bucket, building up irritating, dangerous fumes that can blast you when the lid opens. Similar gas exposure can happen when opening a trichlor feeder, especially if flow is restricted and tablets sit in a damp, concentrated environment. Safe habits matter: keep containers sealed tightly, minimize time the lid is off, open away from your face, and avoid standing downwind.

Finally, shortages and price spikes push people toward alternatives like cal hypo tablets, and that is where the biggest hazard appears: never mix cal hypo and trichlor. Combining them, especially inside a chlorinator where small amounts of water are present, can generate heat, toxic gas, and a violent explosion. Cal hypo is a strong oxidizer and can sustain burning by supplying its own oxygen, making fires harder to extinguish without large amounts of water. The bigger takeaway for long-term pool care is stability: choose a sanitizing strategy that keeps CYA, pH, and alkalinity from drifting so far that every weekly visit becomes an emergency. A stable pool is safer, easier, and cheaper to maintain, whether you’re a homeowner or a full-service pool professional.