I also have a Rex style reactor on my return from my sump tank. Except I have the return going in the bottom and out of the top to the tank. I am pushing about 800gph and I do see the fine co2 bubbles going into the tank.
My thing is no matter how much co2 I push in my drop checker stays blue. I do have it set up on the opposite side of the tank. Is this a good idea or not? Not sure if it makes a difference but I do have lots of co2 bubbles all over the drop checker.