Id say run the factory sponges of course. Then I run one tray of pre filter (Probably won't next time in favor of purigen. Then I run a tray of filter floss. Then two trays of bio max.
So in order from top to bottom my ideal setup in the future will be
Factory sponges
filter floss, purigen , bio max X2 trays.
I have not used a 305 in a while is it three trays or four ?