If the water flows from top to bomm you want the water to hit the polyfil first, then your carbon or you can use another basket of biomedia, and then the final biomedia tray. You want the cleanest water on you biomedia.
This is just how I would perceive it to be though, I run all fluvals and aquaclears so I;m not toally sure on how the rena works, but it should be similar.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.