I had a similar setup. I was using a Magnum 350 and two Emperor 280's on my 75g. I say 'had' because once I planted the tank and added CO2, I got rid of the Emperors (way to noisy). I only run the Magnum with the micron filter, no charcoal, no nuthin.
I had a very small fish load when I started...6 neons, some Otos, 8" chocolate pleco and a few other various tetras. Since planting the tank I have doubled the neons, added 12 bloodfin tetras, 12 green fire tetras, several more Otos, some SAEs, and traded the pleco for a bristle nose pleco (the chocolate pleco kept uprooting my plants). It has been 3 weeks since adding the extra fish (all at the same time mind you) and I have yet to find any measurable ammonia or nitrites in my tank.
I do have a large qty of plants in the tank which, as previously stated, gives more surface area for the bacteria to colonize on. I guess it just depends on your tank, plant load, and fish load. This is just what has worked for me. Good luck.
South Western Ohio Aquatic Plant Enthusiasts (SWOAPE) member