Muriatic acid is used to lower ph. Nasty stuff. You have to be really careful when you use it. (Be upwind so you won't get a bunch of fumes, eye protection is recommended, don't splash it on yourself or on your equipment, liner, etc.) Some pools never need it. My pool's ph is very stable and I never need to lower it. Every once in awhile I need a little Borax -- especially if we get a lot of rain (acid rain) but even then it isn't very often.