Pool water is always lost through evaporation,
The best way to establish whether or not you have a leak is to measure the difference in the drop in height of the water in the pool and the water in a bucket that is placed on the steps of the pool. Make sure that at the start the water is the same height in the bucket as the pool. After several days, if there is any difference at all, then you do unfortunately have a leak. In order to find the leak, start with the water at normal height. Mark that level with a waterproof pen or waterproof tape. Run the filtration system as normal for a day and then measure the drop in water level. Next step is to top the water up to its previous level, leave again for a day, but this time with the filtration system off, and measure the drop again. If there is a greater loss with the filter system on, the leak must be on pressure side of your pipe work, somewhere past the pump. If you are losing less water with the filter system off, then leak is on the suction side of your pipe work, in other words somewhere before the pump. If the leak is in the pipe work, there are two ways to find the leak. First, simply dig to expose the pipework until you can identify the problem. We would recommend the use of professional leak detectors, who will use detection equipment and dig only if necessary. If your water loss is the same with the filtration off as on, then your leak is likely to be in the structure. In a concrete pool, any cracks or damage to the shell are likely to be fairly obvious. To test a crack for leaks, first turn the pump off at least an hour before you start. It is necessary to be in the pool, and squeeze some dye such as food colouring around the crack. If the dye is drawn in to the crack and disappears, then it shows that this is where the leak is. In the absence of any visible cracks, squeeze some dye around any items that pierce the shell such as main drain, returns, vacuum points, lights, etc. Check the mouth of the skimmer where it meets the concrete. This is a common area for leaks. If you have a leak in a liner structure then it can be a little harder to find and fix because the liner hides the structure. Firstly, as above, check around all the items that pierce the liner such as the skimmer, main drain, etc first. Next, look at the fabric of the liner for holes and tears.