I just added an FX6 to my ML C-530 & C220 on my 155gl (5ft x 2ft x 2ft) tank. In the new FX6, I put a lot of the Eheim Substrat Pro in each of the 3 baskets along with a course foam pad on top and a fine foam pad on bottom. The C-530 is chock full of the Substrat Pro also. Matrix in the C-220.
I'll let you know how it works out, but so far I'm very happy with the FX6. Great mechanical filtration to go along with the C-530's huge biological media capacity. Best of both worlds.