Why would you want to go smaller? With the bigger one you could manage if your Emperor ever died and wouldn't have to clean the canister out as often.
Personally, I run a single FX5 on my 75 and it stays clean and well filtered. I also have 2 Rio 1700's running 8 USJ's though.
Personally, I run a single FX5 on my 75 and it stays clean and well filtered. I also have 2 Rio 1700's running 8 USJ's though.