Pool Leaks, below ground are usually best handled by a pool professional with experience in leaks. First make sure you have a leak. Normal water loss due to evaporation, in the summer, in Phoenix, is about 3 inches a week. (Shut off your water leveler, put a pencil mark on the tile at the current level, and see how much the water level drops.  ( See "Unknown Leaks" for more instructions. Click HERE.)

        Keep in mind that that wet spot on the ground may be from an old irrigation line. Make sure that you are losing water before you spend hundreds to have a person tell you, you aren't losing water. Another test is to place a 5 gallon bucket on the top step, put a couple of heavy rocks in it to keep it in place, and then fill the bucket to the exact same level as the pool. If it rains, it will rain in the bucket too. If it evaporates, it will evaporate from the bucket as well. If the level in the pool drops quicker than in the bucket, you have a leak. You can also see how much you are losing.

        If you aren't "losing" (Beyond Evaporation) more than an inch a week and you aren't about to sell your home (There are laws about disclosing known problems.) I might just keep an eye on the problem. If it never gets worse, I might not do anything about it. A small leak is actually beneficial in the Phoenix area where evaporation, and its associated problems, are so high.

        A pool professional can do further testing if you are pretty sure that you really do have a substantial leak.