Hot tubs can be fine places for relaxation, especially after a long day at work or play. However, hot tubs require sanitary water to prevent contamination by bacteria or organic matter. Chlorine is still the most common disinfectant used in pools and hot tubs, though a hot tub does better with certain kinds of chlorine than others. For example, pool chlorine tablets shouldn't be used in hot tubs because they're very high in chlorine, and they dissolve too slowly.
One reason it's not a good idea to use pool chlorine tablets is that they're so high in chlorine strength, at 90 percent. A single 3-inch chlorine tablet will deliver nearly 5.5 parts per million (PPM) of available chlorine in a 10,000-gallon pool. A typical 300-gallon hot tub, then, would receive far too much chlorine from a chlorine tablet. Alternative 1-inch chlorine tablets are nearly as high in chlorine strength and dissolve even more slowly than 3-inch chlorine tablets, unfortunately.
Chlorine tablets are also stabilized, meaning they're loaded with cyanuric acid (CYA). CYA is an effective chlorine stabilizer, but it shouldn't be used in a hot tub. CYA in such a small body of water as is found in hot tubs will greatly slow down the available chlorine effectiveness, preventing it from attacking certain bacteria. For instance, chlorine slowed down by CYA won't be able to attack the bacteria Pseudomonas aeruginosa, which causes "hot tub itch" on hot tub users' skin.
Proper pH levels in a pool or hot tub are vital to the chemical balance health of pool or hot tub water. Chlorine tablets are extremely acidic and will easily drop a hot tub's pH to well below 7 once they're added to hot tub water. The recommended pH level for a hot tub is 7.2 to 7.8, and a pH below 7.2 can cause user discomfort. Additionally, low pH in hot tubs will cause corrosion of hot tub parts.
The best chlorine-specific sanitizer to use in hot tub water is granular chlorine, also known as sodium dichlor or just dichlor. Granular chlorine is nearly neutral in pH at about 6.7 — 7 is neutral — and is able to tolerate the hot water in a hot tub quite well. Typically, it only takes about 1 tbsp., or approximately a capful, of granular chlorine in a 300-gallon hot tub to maintain chlorine levels. Chlorine in a hot tub should be between 1 and 3 PPM.
You usually have to add chlorine to a hot tub about three to four times per week, partly because warm water degrades chlorine more quickly than cool water. Also, pH levels in a hot tub should be checked about three times per week and adjusted as necessary. Hot tub and spa pH increasers and decreasers are only added by the capful as well. Lastly, when adding more water to a hot tub, it's also a good idea to include a little scale control chemical.