For one thing I'm pretty sure the low pressure gauage is busted, the needle never moves. But I've read that it doesnt matter?
This is correct, even if the low pressure gauge is not working, your regulator will still function. It just means you will no longer be able to determine what your delivery pressure is set at.
Basically I plug the regulator into the wall and set the bubbles to 1bpm.
To be precise, you are plugging the solenoid into the socket, but go on...
It works fine. So then I plug it into my pH Controller. It continues to output 1bpm into my reactor. Once the pH in the reactor reaches 6.5, the controller shuts off the solenoid and the bubbles stop flowing. Then when the pH is higher than 6.5, the controller turns on the solenoid and the bubbles continue to flow around 1bpm. This goes on for about a day.
All is fine so far, though, I assume you meant 1 bubble per second (bps), and not bubble per minute (bpm).
The next day when I check, when the controller turns on the solenoid, the bubbles are STREAMING out of the regulator! So much so that the excess CO2 is blasted back out of the 'intake' tubing of my reactor and into the tank (it's literally like if you take a straw and blow as hard as you can into a glass of water).
What is the reason behind this? It's as if pressure builds up while the solenoid is off, and when it's back on, the pressure releases and the CO2 spews out.
I'd hate to just throw this setup out and have to start new. Do you have any troubleshooting tips for me?
Your reasoning is spot on; this occurrence also occurs with many other regulators, but perhaps not with the same violence. Ideally, the streaming CO2 should calm down after a few minutes, after the pressure equalizes again.
It may be that your delivery pressure is set a bit high; try dialing back the delivery pressure (unfortunately, without the low pressure gauge, there is no way to tell how far you are dialing it back) and adjust from there.