I suspected a leak all summer but did not want to shut down the equipment to find it.
I used the brain-dead approach and turned everything off, and the leak was 3/4" plus per day until the water level got close to the return jets, and it has since stabilized.
Next step is a pressure test, but in the mean time I started looking for leak repair companies, and I came across a web site that uses an two part process called flood grouting. ( f l o o d g r o u t i n g . n e t )
Has anyone heard of this process?