the controllers are not absolutely necessary, but are nice to have just in case your pH starts to drop it can be shut off before things get bad. since it says it does not need it, than i am guessing it has a really slow drip rate.
pretty much any Ca reactor can handle any size tank. it is just a matter of how fast the media gets used up. also the higher the flow the greater chance of pH drifting downward. if that is a large water cannister than it should be large enough to keep the flow slow enough to keep the pH high by the time it gets to the system. i think that made sense.
sorry, i did not mean regulator on the CO2 tank, i meant the solenoid on the reactor. you only need one regulator on the CO2 tank, not two.
G~