It's called bleach.

https://modernsurvivalblog.com/survival-kitchen/bleach-water-ratio-for-drinking-water/

 

It's not. The person below your comment got it right.

 

You might be thinking of pool shock. It's used to kill bacteria and algae, get the pool pH to 7.2 and 7.8, and lower the chlorine levels.

 

Bleach is OK if you're filling a perfectly clean container to the top, and not opening until you are to use. If you let germs in, you have to start over with fresh bleach, because it expires. BLEACH EXPIRES, but if the water is clean when sealed, it's still clean when opened.

Some forms of chlorine can be dangerous to store.

I believe what you saw was Sodium dichloroisocyanurate granules, if you are looking for ongoing purification.

 

I believe you're thinking of pool shock. Has to be at least 73% calcium hypochlorite. I think it's one teaspoon will purify 100 gallons of water. If anyone knows better, please speak up.

 

Leave that stuff alone. It demolishes metal and dissolves almost all plastics and rubber.

Good luck finding an all glass or all teflon container.

It's also a fire hazard. It's about as fun to keep in the house as uranium yellow cake.

Legions of SB forum members have experimented with trying to safely store it long term.

We have many threads you can search for that explains the aggravation and futility.

There are better and safer shelf stable chlorine products than the pool chemicals.