Leak vs evaporation?

I'm still a newbie - we have had the house with pool just over a year, and I am still learning about it. Our pool has a spa that flows into it. We run the filter from 8 to 5 daily. At night, the spa shows a considerable water drop (like 7" for the 7' diameter spa) - which is then replenished each morning with the float water filler. I'm not sure whether this water drop represents the collective evaporation loss over the entire pool + spa surface area, or just the spa. So, I'm unsure about how to do a bucket test for evaporation, and whether to focus on the spa or pool (or both) for leak detection.

I'd appreciate any guidance/pointers/advice from all of you more experienced pool people. Thanks.

7" is a lot and could be leaking but it could also be draining from the spa into the pool if the spa is elevated. Is the water level in the morning at the same height as the pool or is it below the level of the pool?

The spa level is still higher than the pool level in the morning. Maybe a check valve issue?

That's what it sounds like. If you have the type with the removable cover, you can inspect the valve itself.

Evaporation bucket test for swimming pools:

1. Get a clean bucket you can suspend in the pool.
2. Fill it with pool water to within 3 - 5 inches of the top.
3. Suspend it IN the pool, so that the pool keeps the bucket at the SAME temp as the pool water. (can use steps, on some pools)
4. Mark water levels in both bucket and pool.
5. Compare at 24 and 48 hours. If the pool loses much more than the bucket does . . . you have a leak.