Here's my two cents. You don't need a separate pump. I just built a nitrate reactor using Seachem's deNitrate media.. We'll see how it goes, maybe I'll pack it with sulfur instead. Anyway, I have a single loop, which goes through a canister filter (FX6), then another pump (blue line 40), then a chiller, and returns to the tank. For the reactor, I made a T-junction before the chiller, and another one after the chiller. So from the first T-junction, I have a ball valve, then a needle valve, then a flow meter, and finally it goes into the reactor. It comes out of the reactor into the other T-junction, right after the chiller. There is enough head loss through the chiller to produce enough flow through the reactor, and I'm able to dial it in using the needle valve. I currently have it set to 0.5 GPM.
I can post a picture if you want, let me know.