How to Choose What Type of Chlorine and Cyanuric Acid to use for Your Pool
Choosing the type of pool chlorine:
Chlorine, which kills bacteria, algae, and microorganisms, is available in bottles, 3-inch tablets, 1-inch tablets, sticks, and a granular form; however, upon inspecting the labels, you will see that the active ingredient is exactly the same in all of them. Despite the wide range of prices, the only real difference you may find is the concentration of the active ingredients. The active ingredient in 3-inch tablets, 1-inch tablets, and sticks is called "Trichlor" (or Trichloro-S-Triazinetrione), and the active ingredient in granular chlorine is called "Dichlor" (or Sodium Dichloro-S-Triazinetrione).
The most common (and therefore the least expensive) form of chlorine is 3-inch tablets, which are slow-dissolving and require less maintenance. Chlorine sticks are larger and dissolve even slower than 3-inch tablets but are not as popular. 1-inch chlorine tablets dissolve more quickly than 3-inch tablets or chlorine sticks and are better suited to above-ground swimming pools, small in-ground swimming pools, and spas. Look for a concentration of 90% Trichloro-S-Triazinetrione in chlorine tablets or sticks.
Note that cheap, "big box" slow tabs and sticks tend to have binders and fillers that keep the tablet together. You will notice the difference as they dissolve: cheap tabs and sticks tend to crumble or fall apart within 2 to 3 days as opposed to gradually dissolving and maintaining their shape.
Granular chlorine works just as well as the tablets and sticks mentioned above; however, inorganic chlorine such as calcium hypochlorite must be pre-dissolved in a bucket of water before adding to a swimming pool. It must also be added to the swimming pool almost every day. Other types of organic chlorine (Sodium Dichloro) or inorganic Lithium Hypochlorite do not need pre-dissolving. These allow very precise control over the chlorine level of the swimming pool but require daily testing and addition of the chemical. Look for a concentration of 56% to 62% Sodium Dichloro-S-Triazinetrione in granular chlorine.
Choosing cyanuric acid:
Use cyanuric acid with caution. Cyanuric acid (CYA, also called isocyanuric acid) is found in dichlor / trichlor tablets. Although cyanuric acid is a stabilizing ingredient in chlorine that prevents it from being destroyed by the sun, it does so at the cost of reducing the effectiveness (ORP, or oxidation reduction potential) of the chlorine. If you do use cyanuric acid, be sure to test the levels. If the levels are too high, the chlorine will completely lose its sanitizing ability.
Certain new studies are showing that CYA really needs to be maintained at a level no higher than 40 ppm allowing chlorine to perform optimally (high levels of CYA contribute to TDS or Total Dissolved Solids which "interfere" with chlorine activity).
If you choose to avoid cyanuric acid, look for calcium hypochlorite (solid) or sodium hypochlorite (liquid). You should also make an extra effort to test your pH; these two chemicals contain strong bases and will raise pH if used in sufficient quantity. Using liquid chlorine will also help you continue sanitizing your pool without adding to your Cyanuric Acid levels. Cyanuric Acid = Stabilizer. Stabilized chlorine (tablets and granular) will contain high levels of Cyanuric Acid.