I was having the same problem with the BRS single reactor. I would get a nice rolling tumble, but by the third day it would be clumped. I was considering changing out the pump that came with it but was told that the Mj1200 was plenty strong enough for that reactor. I tried mixing GAC with the GFO, but I feel I was throwing out the GFO before it's time because the GAC was depleted and the GFO wasn't. I read some good and bad things about Phosguard, but made the switch anyway because that doesn't need to be tumbled in a reactor. I have been using Phosguard for about 7-8 months now with no ill effects on my tank so I'm sticking with that.