Swimming Pool Blog

Cal Hypo Tablets vs Trichlor Tablets

Hayward Pool Heater Troubleshooting
by Mark Garcia March 15, 2015



For pool chlorination, there are two main types of chlorine - hypochlorites or isocyanurates. Calcium Hypochlorite (Cal Hypo) and Sodium Hypochlorite (Bleach) are the former, and Trichlor tablets and Dichlor shock make up the latter.

All types of pool chlorine produce hypochlorous acid, the killing form of chlorine, but all have different "side effects" to their use. 

  • 10 ppm FC from Trichlor raises Cyanuric Acid (CYA) by 6 ppm.
  • 10 ppm FC from Dichlor raises CYA by 9 ppm.
  • 10 ppm FC from Cal-Hypo raises Calcium Hardness (CH) by 7 ppm.
  • 10 ppm FC from Sodium Hypo raises Salt levels by 17 ppm.



Sodium Hypo (liquid chlorine) is a popular choice where available, however due to the very high pH (13) of liquid chlorine, a gallon of acid is required for every 10 gallons of bleach used, and maintaining constant pH and alkalinity levels can become difficult. Liquid chlorine systems also require a safe space for large chlorine vats, and close access for a bleach delivery truck.

Cal Hypo (granular or tablets) are another choice for large pools with high chlorine demand. Granular has a fairly high level of pH (11), but tablets are lower (9). It won't contribute to cyanuric acid or salt levels (sodium), but it does increase calcium hardness levels in the pool, which could be undesirable for hard water areas. The tablet form can also be mixed with additives to help clarify the water and reduce stain and scale formation.

Trichlor (3" tablets or sticks) are not usually used for large commercial pools, as they don't dissolve fast enough to meet the chlorine demand, but are more suited to pools under 50,000 gallons. Being stabilized, Trichlor tablets will build up cyanuric acid in pools over time, requiring periodic additions of fresh water. Having a very low pH level, Trichlor tablets depress the pool water pH and alkalinity levels, requiring frequent additions of pH increaser.


Sodium Hypo (liquid chlorine) is delivered via trucks that also supply dry cleaners and industrial users of bleach, and pumped into 50 gallon vats. For smaller pools, liquid chlorine can be hand fed daily, or 5 gallon drums can be used. Chemical pumps are used to pump from the chlorine vat or drum into an injection fitting placed into the return pipe, and have a dial to control the chlorine flow amount.

Cal Hypo granular is used by either manually broadcasting or dissolving into water and pouring into the pool, or it can be used with a granular cal hypo feeder. Cal Hypo is also made in pellets and pucks, or tablets of various sizes, designed to be used in very specific feeders, usually made by the tablet manufacturer, although they could be used in floaters, or floating chlorinators. Most cal hypo feeders like the Accu-Tab or the Pulsar are expensive and complicated items, with exception to new models recently introduced, such as the CCH cal hypo tablet feeder.

Trichlor tablets are used in a typical residential chlorinator, installed onto a return line, holding up to 9 lbs of tablets, and can also be used in a chlorine floater. Dichlor can be dispensed in a granular feeder made specifically for Dichlor, such as the Watermatic dichlor feeder, and although more pH balanced, is one of the most costly feeders and types of chlorine to use. In many pools, build-up of cyanuric acid is a concern as it reduces the efficacy of chlorine and requires periodic draining and refill with fresh, unstabilized water.


Cal Hypo tablets may make sense for you if you operate an indoor pool, or a not too large outdoor pool with problems with cyanuric acid levels rising too fast, and you don't have extremely hard water in your area. 

Thanks for Reading!
Mark Garcia