Muriatic. Go to pool shop and get a tile calcium remover. It will be a thick gel containing acid. Scrub, dwell, rinse. Some are just really bad and need to be polished with a medium grinding wheel.
Unless water is already out of balance, that bit of acid wont make the water fall out. If its decent sized pool. Thats why you get the gel. Like roof washing (cling on, roof snot) it makes the acid cling to vertical surfaces.
