Are you in Ocean Forest? If you are, I'm finding that low to mid 7s with tap water going in puts me close in range for PH out. The out PPM will go down a little after a few watering's to run off. I'm sure we are probably getting the same tap water.
I started having the same issue with my exhaust fan a few days ago. I'm now loosing neg. pressure in my rent. These inline booster fans don't seem to be cutting it now that the carbon filter is getting restricted a little. With the filter disconnected the fan is loud as hell. Need to get a speed controller for it to slow it down to run without the filter. Now wish I would have spent the extra on the good exhaust fan.
When I set my next tent up (already thinking about another one LOL), I will get the good exhaust fan with a speed controller, and not hook the filter into the system until the girls start to stink. Really no reason for a filter until then. This way these expensive ass filters will last a lot longer.