I just started using the GFO/Carbon reactor from Bulk Reef Supply. It uses refillable cartridges in an RO style filter housing. There is a single filter version as well that you can run GFO and carbon in the same cartridge.
With the foam in-between the GFO will tumble and the carbon doesn't.
They recommend a MJ-1200 for the twin version but I have it teed off the return line.
I think it's pretty slick, I was tired of fighting with the lids and connections on the Little Fishies reactors.
They have a video on their site showing how it works.
