My guess would be that you have too much flow through your media chamber. The flow should be gentle through the ARM media. Meaning, your CO2 saturated water should flow gently through the media, and not cause it to move. There should be no tumbling of the media. I use an Iwaki MD20 as my recirculation pump on my reactor, but I have the flow choked down with a ball valve. The pump is capable of around 500GPH, but I'm guessing I have it choked down to maybe 200-300 GPH, about half flow. The ARM media in my reactor does not move, and it shouldn't.
Also, I'm curious where your effluent is coming from in your reactor. Do you have a picture? It should come out near the top of the chamber, so that gravity works in your favor to keep any particles out of your effluent. I have my effluent coming from the pressure side of my line, so it's a little bit farther away from the reaction chamber.