J2theD:
i too, have a 75 gal. community planted tank.
I also use a fluval 405
canister filter.
On my tank i have 2- double bulb, 4 ft. light fixtures. There was room for them to fit side by side across the top and still be able to open the lids for feeding.
The lights i use are expensive bulbs from the pet store, and i have the spectrums alternating. By this i mean, when you buy bulbs for your fixtures, some give out a spectrum making the bulb appear red, and some bulbs appear blue. My bulbs are alternating red, blue, red, blue... making sure my plants are getting a good dose of each spectrum!
I have pictures on my profile of this 75 gal community planted tank if your interested in seeing how good its doing, and how good yours could also be. Ofcourse setting your tank in a window would greatly increase your light too, when my plants were at their thickest density i moved the tank so that it was getting direct sunlight in densely planted area. This is good for alot of plants- bad if you dont have that density. Without many plants direct sunlight will greatly increase your temp. & give you blooms of green water algae for what seems to be never-ending!
hope this helped if you'd have any more-specific quesitons pm me