What do you think of their idea of splitting a single unit with GFO in the bottom and carbon in the top? Think that would work for a 90 gal?
Carbon has to be changed out more than GFO but for me I error on the side of caution and change out both on the 1st of every month. I run 2 bags of regular Chemi-Pure for my carbon, and GFO in a reactor. I have figured out GFO on my tank usually runs out around 45 days were as the carbon runs out just shy of 30 days. Every setup is different though, and I feed my tank heavily, so my GFO runs out a little faster than on some other peoples tanks.