If you're in the habit of removing and replacing most of the tank water weekly, why do you need more filtration? The filters are just moving water that's already clean. I have a 100 gallon tank with one dual sponge filter to aerate the water, that's it. I change most of the water weekly, so there's no need to over filter the tank. You can do the same, as long as you're an aggressive water changer. If you're not, then you'll need several filters.
"Fear not, my young apprentice. Just change the tank water."