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 tends 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.