Calcium hypochlorite, better known by its nickname of cal hypo, is one type of product commonly used to treat swimming pool water. It comes in various forms, such as granular or in tablets that look like hockey pucks. Because of its solid, concentrated form and ability to kill any number of pathogens while keeping water clean and sparkling, it is considered the most ideal product to choose when it comes to sanitizing swimming pools.

Cal hypo’s solid form makes it is easy to transport and to store. It can be purchased in small bags that are easy for any homeowner to handle and use. It is inexpensive, fast-acting, and very strong. If it’s added correctly, it will not damage the pool or equipment. It is also used to shock pools, which means giving the pool a high dose of chlorine to quickly eliminate any build-up of combined chlorine and extra germs that may have manifested from a high bather load or rain storm.

Cal hypo does not contain cyanuric acid, which is a chlorine stabilizer, so cal hypo’s ability to kill germs quickly is not affected by overstabilization that can slow it down. Too much cyanuric acid can also lead to algae blooms, because the chlorine can’t kill the algae as fast. On the flip side, cal hypo’s lack of cyanuric acid also means chlorine is burned off quickly in the sun, which cyanuric acid’s job is to protect from happening. However, cyanuric acid may be added separately to the pool to keep this from occurring.

“It’s really hard to control your chlorine concentration with an ORP controller in the presence of cyanuric acid,” says Ellen Meyer, product safety and government affairs manager at Solenis. “It really helps not to have any cyanuric acid present.”

There are other methods of treating a pool, such as trichlor-s-triazinetrione, or trichlor for short. Trichlor is another popular product and is very similar to cal hypo in that it comes in tablets, is inexpensive, easy to transport, and has a high amount of chlorine. However, unlike cal hypo, it does have cyanuric acid to protect the chlorine from being worn away by the sun. Each tablet typically has about 50% cyanuric acid. Cyanuric acid doesn’t dissipate over time like chlorine, so after a while, your pool can end up with a high cyanuric acid level, which can only be lowered by partial draining and refilling.

Trichlor also has a low pH, which can cause pipes to corrode. “Typically a lot of pool builders will tell you don’t ever put a trichlor tab in your skimmer because you can have damage to your downstream equipment,” Meyer says. “Cal hypo has a high pH, so even though you have a high concentration of chlorine, it doesn’t corrode your heater or any of your other equipment if you put it in either a skimmer or feeder.”

Sodium hypochlorite, also known as liquid bleach is another option, but it is only sold as a liquid which makes transporting difficult due to the weight. It is also extremely corrosive, so it must be handled with much more care than its solid counterparts. In addition, liquid bleach loses much of its strength in storage and at high temperatures.