How are you adding Ca and Alk today? Assuming a two part, how much are you dosing. I would be very careful with a reactor with a low Ca Alk load.
I'm not familiar with that reactor, but typically you need a pump to supply water to the reactor. The pump on the reactor frame is probably just a recirculation pump. It looks like from the photo that your water into the reactor is hooked to the suction side of the pump. You need to be careful of that, if your pump shuts off, the reactor may drain back into your sump.
I have a "T" off of my main tank pump. This gives me a nice constant pressure to the reactor. Water enters high in the recirc tubing, so it cannot syphon back. The return line has a needle valve to control the effluent. And this line just goes into my sump in a convienient spot, which is downstream of my tank PH probe.
You'll probably also want to MOD the reactor to control the pH in the main reactor chamber. If you don't have a ph controller, if the water flow quits moving tank water into the reactor, you can end up with a very very low pH in your reactor and turn the media to mush. Also, I think you'll find that most people that have been running a reactor longer don't use the find media.