Pool and Spa
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It sounds to me like you have a leak in a return line. When the pool is off you have just stagnant water in the lines, and the leak is not apparent but when you turn the pool on you are pushing water through the lines and if it is a small crack it is most likely pushing the water out the crack and that is why you notice the leak only when it was running. When someone only looses water when pool OFF it is usually a suction line. I would say to hire a pool professional to pressure test the return lines to find out what line is leaking. The hydro stats would not change the leaking with pool off.