We bought a house a few weeks back with a 27000 gallon inground pool and spillover spa. Pump/filter are located a little below pool water level, determined from my eyeball estimation (and the fact that I removed the lid to the pump basket and water poured out until I got it back on.
Here is what I've observed so far with pump running 24/7 on pool/spa combination return....
Water and salt levels have been dropping in the pool since we moved in -- I was adding 1/2"-1" of water per day. Air was seen in pump basket, was able to clear it by isolating suction line to skimmers only, eliminating main drain. This did not stop water loss, so I put plugs in the 2 main drain lines -- still did not stop water loss.
Turned off return to spa, so that pump runs 100% pool for the last 2 days. Spa level dropped quickly, now below bottom return jets and just above spa drain (it has probably not stopped dropping yet). The spa water level is also below pool water level, but not sure if it is below valve level -- that would just be a guess. In this mode with spa return off, the pool level looks to have stabilized other than possibly evaporation.
I would like to unplug the main drains at some point to make sure there aren't any water leaks there, but want to figure out where the water is going first. Here is a picture of the pump/filter, labeled with my guess of what the valves and pipes are =)