I figured its the xp3 when you mention trays. Yes, Renas can be used as a reactor, but I would still put the media the mesh bag though. they might travel through and damage the impeller.
the particles will get smaller when they tumble inside due to water flow, and small enough to get through the holes in tray cover and into the impeller.
i bought a little 10 inch canister in petgoods for ten dollers for my phosban it works great brings two sponges my phosphate is super low and tumbles perfectly i also saw it in brs for ten dollers