You made me double check mine. I had the same battle a few weeks ago. I kept slowly tweaking the bubble count until it was stable. Now may solenoid has been on for 5 days! I keep mine between 6.7-6.9
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Sorry , I should have been more clear.
I use a Milwaukee reg & solenoid w/bubble counter . Glycerin in bubble counter . 1/4" needle valve for effluent. I have a Geo 612 reactor & run at a PH of 6.5.
Also just added a second chamber , so effluent runs thru vertex reactor body filled with media & overflows into sump - trying to raise tank ph , seems to work.

Ah ok, that makes more sense, thanks!

I started off with a Milwaukee regulator/solenoid/needle valve/bubble counter combo but it's now been modded enough to be its own beast. Both the needle valve and the solenoid are swapped out now. I was able to unscrew the bubble counter from the needlevalve and I use a 1/8" hose barb from an old wooden airstone as the input, and it's just mounted to my wall next to the reactor.

I've been playing around with CO2 flowrate since yesterday and I'm getting closer to steady state but still not really there. The bubble rate isn't a nice steady rate, it goes more in bursts and then has a few seconds of nothing, which I think is probably from the check valve, so that's probably OK. Over a long enough period of time the bubble rate is still consistent. My challenge now is that it really doesn't take much of an adjustment (like, seriously, a small touch of the valve) to go from "the CO2 is too fast and thus the solenoid cycles off and on too much" to "the CO2 is too slow and the despite the solenoid being on the pH is just climbing higher and higher away from the desired set point".

I'm not quite sure how I can make this better. I guess one option is lower the regulator pressure and hope this gives me a bigger sweet spot for adjustments; and another is drill the reactor and get the pH probe inside the reactor chamber itself (right now the reactor effluent empties into a little capsule that the probe sits in - so the pH measurement is actually quite slow to react to whatever is happening inside the reactor).

The more I think about it the less I like not having the probe inside the reactor, so that's probably the next thing to fix..

What psi do folks tend to have on the regulator second stage?

I'll double check but I think 18PSI

I tend to have the opposite thinking, I'd rather it cycle more often than stay on for long periods of time. IME these solenoids typically fail off since they are NC and when left on for long periods they produce a far amount of heat. So seems to me having it stay on for longer periods is more likely to create a failure, especially one with it left open. I use to run my bubbles fairly fast for this reason, the valve would only kick on for 30 seconds or so before shutting off and then would stay off for longer periods until the pH recovered. While this did mean the reactor pH would drop a bit more than you guys are seeing it still maintained near perfect levels in the display.

I also seem to recall setting my target pH lower, closer to 6.4 as suggested by the media, which allows for a slower effluent and more efficient use of the C02.

That's interesting, thanks, I'll check the temperature of the solenoid when I'm home from work.

Mine seems to be settling out at 6.5 now although I was sort of hoping to target 6.6. If you ran fine at 6.4 then maybe I'll leave it as-is for now until I have a more substantial calcium draw (all I have are frags so far).

Here's a chart of the last 12 hours or so:
Attachment 15672

You can see that overnight I still had the bubble rate too fast and the solenoid cycled off/on until I checked in the morning. I did a *very slight* decrease to the CO2 flowrate at 8am (literally, I wasn't even sure if I had moved the knob) was all it took to keep the solenoid to stay on, except that the controller kicked it off when it hit 6.55 (the previous off setpoint). So I lowered the off point to 6.5 to see where it would settle and it seems to have coincidentally completely settled on 6.5 so the solenoid hasn't turned off yet.

I wasn't sure if I was going to try slowing the CO2 again to see if it would settle at closer to 6.6 or if I should lower the off point to 6.45 (I'm a little worried that if it settles right on the borderline then it might switch the CO2 off and then it will go all the way back up to 6.7 before it turns on again ... although you can see by the "up curves" that it doesn't take very long to raise the pH).