If your spa water continues to drop over a period of time AFTER your filter pump turns off your issue is related to your check valve for the spa. Check valves are used to stop the flow of a body of water that sits above the equipment. In all cases, always make sure there is NO DEBRIS that may be causing a small leak. Leaves, palm branches, etc. can often lodge themselves in the check valves causing small leaks that can eventually drain a spa.
If you have a JANDY CHECK VALVE your best solution is to ID the check valve
and replace the flap or combination flap & lid as well as the o-ring (don’t forget to lube) that lies between the lid and the check valve housing. See link for visual http://bit.ly/nbWjBl .
You may, however, have a ORTEGA CHECK VALVE. In this case, the seat assembly (#2) or the spring (#3) may need to be replaced. See link for visual http://bit.ly/r4hLMH .