It is an overflow line, but it was installed wrong and will siphon off approx a 1/4 to 1/3 of the tank.
Thought Redwood fixed this early problem, but beginning to look like with the change over of personnel some of the original problems are re-surfacing (not really a good sign for RW)
Any way, yes just put a small shut-off on that line, just remember to open it back up once you've used some of the water in the tank.
This is getting really sad, that problems that had been reported and fixed some time ago are now coming back to life. Sounds like some of the fixes were never documented and new people now working from the old info recreating the same problems.
Michelle & Ann
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