In the FX6 on my slightly overstocked 75(6 RD's - 10 inch male(DAD), 6 juvies(Dad's babies) at 4-6 inches each):
Top Tray: Pond Matrix
Middle Tray: Substrat Pro
Bottom Tray: Pond Matrix
Stock sponges around circumference.
I also have 2- supplemental 300GPH HOB's just for additional mechanical, plus I have 1 pouch of purigen in each. I have 2 supplemental (koralia knock-offs)powerheads that I cycle on twice a day to push the main fish waste to the FX6 intake.
I also dose rid-x once per week at 1 teaspoon per 75 gal, stirred in jar of water for an hour or two, and strain the bran through a brine shrimp net.
So far so good as currently set up, but I have quite a few unused spare can's I can install if I need to add additional bio(1-FX4, 1-FX5, 1 additional FX6, 2 Sunsun304B's, 1-SunSun302, 1-Cobalt EXT, 2-Magnum 350's, and 1- Eheim 2262(bought for a 300gal project that hasn't materialized).
I'm currently considering adding the sunsun302 and feeding the return to a "modified" Phosban reactor filled with purigen to try to keep nitrates lower(in an attempt to adsorb\absorb nutrients before they can be converted to nitrate by bacteria). Unfortunately, I just haven't had the time to set up and test.