If you routinely need to add more than two inches of water to your pool per week, you may have a leak. Your pool will lose some water naturally to evaporation, some to splash out and some to backwashing your filter. Many leaks are not detectable. Thanks to state-of-the-art technology, most pool or spa leaks can be found and repaired without major disruption.
If you determine that your pool is losing water, turn off the filtration system and note where the water stops dropping. Vinyl Liner pools need to have water in them at all times! If you have a liner pool and the water level is dropping rapidly, start adding water and call a pool professional!