If the water level in your in-ground pool is dropping, there is a good chance your pool has a leak. All swimming pools will lose water from evaporation, but if you notice the water level being much lower than normal, chances are good that something is wrong. Though you could call a swimming pool specialist, you can certainly try and find the leak yourself. There are a few spots in particular that lend themselves to leaks that you should examine first.
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Pressure-side Return Line Leak
If your pool seems to be losing water only when the equipment is turned on, there may be a leak in the backwash or return line. Turn on your pool's filter pump and examine the pipes or plumbing of the backwash or waste line for leaks. Small drips can quickly turn into huge, spraying leaks. Replace any plumbing on the backwash or return line that is leaking.
If the pool seems to leak only when the equipment is turned off, you may have a leak in the suction-side plumbing. While the pump is on, the vacuum created can cause air to be sucked into leaks, preventing you from seeing them. Check the pump basket for air build-up. Also look for air bubbles being emitted from the return lines, as well as air inside the filter tank. Try to remedy any leaks by replacing the pipes, as well as placing a new O-ring on the pump basket lid.
If you have a crack in the tile or floor of your pool, water can leak out quickly. The water from in-ground pools will typically leak straight down into the water table below the pool, and you are unlikely to ever see it. Vacuum your pool to remove all debris, and use a diving mask to scan the bottom of the pool for cracks. Seal any leaks with pool putty.
One of the most common leak sites of in-ground pools is the area where the plastic skimmer compartment meets the concrete wall of the pool. Check around the area where the plastic meets the concrete for cracks or chips. If you suspect a leak, squirt a small amount of food coloring into the water directly in front of the area. If the dye is sucked into the wall, then you've found the leak. Cover the leak with a layer of pool putty.