For a 100 gallon tank you would want at least two Magnums. If for nothing else to avoid dead spots. The one good thing about Magnums is they make awesome CO2 reactors by themselves. Just put the CO2 into the intake and away you go.
I'm not fond of the Magnum series as filters as they have a very low media area for the amount of water they pump. They load up very quickly IMHO. I prefer cannister filters. A Rena XP3 would be a great addition to your tank setup.
I'm a big fan of water movement. On my 55 gallon I have a Rena XP3 with spray bar, a 170 gph power-head, and a Bio-Wheel Pro 30.
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