If water fills up the spa when the pool pump turns on and then overflows into the pool and causes water loss each time the pump turns on what can be done to stop this?

Answer

It sounds as if one pump is filtering both the pool and the spa.

Check to see if you have a three-way diverter valve in the Plumbing after the filter. These types of adjustable valves are sometimes called "Jandy" valves, because they are made by the Jandy company. Go to their Website to see what one looks like and how it works. Make sure you turn the top small knob counterclockwise to unlock the valve first, then click the valve handle around until the deisred flow is reached with the pump running

You should be able to divert enough water flow from the pump to a bypass, or return, that will keep the water from overfilling the spa when the pump is on.

Or, check to see if the three-way valve is in-line to the spa plumbing, it will still need to operate in a manner that will maintain the spa level. The feature that you are seeing is used to flush out the spa so that all of it's water can be filtered into the whole volume of the pool, then replaced again by the pump -but the sap should not go empty or overfill while doing so.

Some of these valves have electronic solenoid controls to open and close them. Your controller may just need to be programmed to "tell" the valve to open and close when needed.