1 per 9 seconds sounds like a really low bubble rate unless your flow rate is a slow drip (which for your tank size should be greater, if you have at least moderate Ca uptake), or maybe the high psi is making up for the low bubble rate (I'd lower the psi to less than 5, 2 or 3 works for me). I have the same setup, though with the 618 Ca reactor (200 gallons total system volume) and run my reactor at about 1 bubble per 1 or 2 seconds and at about 3 psi. Since I've been dialed in for the past several years I actually adjust the psi on the regulator to hit my target pH level, which is 6.6. The API regulator is kind of touchy at the low end and very easy to go too far one direction or the other, so I find the psi gives finer control (a half psi one way or the other is all I change). I've never been up around the high end of the bubble count on the API, maybe since I run my psi fairly low. I'm not sure what the flow rate is on my effluent, but it's a steady stream within 1 inch of the outlet (I use one of those micro ball valves at the outlet and black vinyl tubing to prevent growth/clogging - best to use opaque tubing everywhere with 1/4" tubing IME), then a fast drip at greater distance. You may want to recalibrate your Ca reactor pH probe; I wouldn't think you can get the pH low enough to dissolve much of the media at a bubble rate of 1 per 9 seconds.
Set your Apex "THEN OFF" statement about 0.05 pH units below your target pH, set the effluent flow of something around a steady stream/fast drip (since you have the 624 the effluent flow may be able to go higher), lower the pressure to around 3 psi, and a bubble rate near 1 bubble per 2 or 3 seconds. See where it lands after a day. Then adjust the regulator bubble rate, or if close, tweak the psi. Your Apex should almost never shut off the solenoid that way, only when there is a clog or other problem. Once you get it dialed it they are remarkably stable - the most stable thing in my setup.
