Slow draining shower

[oldSalt]

Very slow draining!!

The last 3 nights or so while showering we've noticed that water builds up to about 3/4 inch above the drain. Last night I let the water run for 20 minutes and it maintained the 3/4 inch level. It took about 3-4 minutes for it to drain completely. I don't believe there is a partial clog as I have (very gently) used a small plunger on the drain. The bathroom sink is working fine. I keep the grey water tank closed and empty it every morning or so depending on usage of the washing machine. I know I can leave it open, and do on occasion, but prefer not to.

I checked the high point anti-syphon valve; but if it was shot all the drains would have trouble emptying.

Any suggestions? Thanks in advance.

Jim