Pumice stone for tiles, vinegar for liners.

You need different tools for different surfaces. For ceramic or porcelain pool tiles, a wet pumice stone is your best bet — just keep the stone and tile wet, rub gently, and the calcium comes right off without scratching. For vinyl liners, never use abrasives. Instead, try a diluted white vinegar solution (1:1 with water) and a soft cloth or sponge. There are also commercial calcium removers made specifically for vinyl — follow the label.

Test any method on a small, hidden area first. And if the deposits are really stubborn on a liner, drain the pool a bit and use a plastic scraper carefully. Do not use a pumice stone on a liner — that will ruin it.