How do you remove calcium and hard water buildup on vinyl pools?

If the pool is full, one of the easiest and cheapest ways is to lower the pools pH and Total Alkalinity below the acceptable range making the water slightly acidic. For the next week (or until the scale is gone) brush down the entire pool twice daily. The idea behind this is water naturally tries to balance itself out; if it is lacking carbonates and bicarbonates (the scale on your walls) it will dissolve it trying to get those levels back in balance. After the scale is gone, keep your water in balance and add a stain and scale preventative weekly.