Easiesy way to find a leak is, close your skimmer valve or valves and only pull from the main drain. Let the pool drain down under normal operation WITHOUT adding water. The water will stop at the leak. If it stops just below the skimmer thats where it is, if it continues to lose water and stops just at a return line then there is a problem with a return line. If it continues to lose water then it most likely is in either the main drain, or the light conduit.