I have been using the standard 1/4 poly tubing for my CO2 supply for over 18 months without problems. I don't think you need special tubing for this application.
I also have been running a Milwaukee CO2 regulator/solenoid/bubble trap. About 2 months ago, it stopped working. It is out of warranty so I decided to try and figure out what went wrong with it to see if could fix it (I had nothing to loose). What I discovered was that the passage from the bubble counter back to the needle assembly was plugged with corrosion and what looked like calcium deposits. I backed the needle out as far as it goes, removed the bubble counter assembly and cleaned out the passage with a 1/16" drill bit. It works like a charm. I don't know why it corroded, I only use RODI water in there... DO NOT use sea water in the bubble counter as it will corrode the brass. If you have water backing up from your reactor and filling your bubble counter, you have a leak somewhere between the reactor and the solenoid. On my setup, water used to backup about 2" up the line. When I cleaned out the bubble counter passage, I used Teflon tape when I re-assembled the unit and water no longer backs up the CO2 line.
The reason people use a pH controller on the effluent of the reactor is to make "dialing-in" the reactor easier and for a fail-safe. There are 2 variables at play when dialing in your reactor, the rate of CO2 administration and the flow rate through the reactor. Without the pH controller, changing the flow rate, changes the pH levels in the reactor (assuming constant CO2 bubble rate). IME, maintaining a consistent flow rate through the reactor is difficult without the use of a metering pump. Every few days I would notice the flow rate dropping and I would have to bump it back up to maintain what I wanted. The problem was that if I didn't adjust it every few days and the flow rate dropped too low, the pH level in my reactor would drop so low that the media turns to mush. With the pH controller, you set the pH and if the flow is cut for whatever reason, the CO2 administration will be cut. This is also why I have my pH probe measure the pH of the water in the reactor, not the effluent. This way when the flow is cut off, (supply pump or needle valve gets plugged) the CO2 will be cut off before the reactor media is turned to mush. A cheap in-line probe holder is a 1/2" John Guest fitting with the tubing stop drilled out of it. This fitting is sold at HD. You can use a second pH controller to cut the feed pump to your reactor if the pH in your tank gets too low.
Easy Calcium reactor Dial-in guide if using a pH controller to maintain pH in the reactor.
1) Calibrate your pH controller probe according to manufacturer's directions.
2) Set flow rate through the reactor to approx 1ml/min per gallon of system water (100 gallon system would have a flow rate of 100ml per minute).
3) Set the pH controller to maintain a pH of 6.8 in the reactor.
4) Measure and record the calcium and Alk levels in your system.
5) Use appropriate additive's to get CA to ~400
6) Let the system run for 48 hrs.
7) Measure CA and Alk.
If still at 400, you are finished, proceed to step 9
If it dropped, drop pH in reactor .1 pH
If it raised, raise the pH in the reactor .1 pH
8) Repeat steps 5, 6, and 7 until CA is maintained
9) Measure CA and Alk weekly and adjust pH if necessary.
