I bought a redhat solenoid from eBay for only $5 but ended taking it
off in the long run. I use a power reactor and have that connected to
my light timer, the CO2 is on 24/7 but it's no big deal seeing it's
cheap to refill and lasts a long time. I also bought my tank and
regulator from eBay ($35) but my local fire extinguisher company who
refills it for me also had some really good deals on used tanks and
regulators. I used the Clippard needle valve to regulate the CO2, not
the best in fine adjustment but it works and so far it has held a
steady rate, just needs a little fiddling around with at first to get
the bubble rate you are looking for. The main reason for removing the
solenoid was to eliminate some cables under the tank and at the same
time I wasn't thrilled about the heat it generated (even though the
heat probably helps dissolve the CO2 better). If you plan on using a
PH controller then it's a must, otherwise you can always put your
reactor on a timer and save some money.

Hope that helps
Giancarlo Podio