My Amazon swords grew and were plenty healthy for about 8months under
low light (30W for 55 gallon) without CO2, ~78F, 7.6pH and zero
nitrates & ammonia. I have several other types of swords now, but the
setup is completely different. Make sure you are giving the swords
substrate fertilizer. That's the best I can offer for you. Nutrients in
the substrate is one thing my swords had plenty of.

I'd forget about the carbon (ya don't need that stuff) change about 30%
water and change 25% once every two weeks with the correct dose
conditioner. Any more and you'd be changing the water chemistry
radically (that's bad if you have fish). Unless you have a problem with
the tank, water changes that often aren't required.

In a heavily planted tank, are filters really required except for
asthetic purposes?

my aquarium page, info and pics at:
www.geocities.com/spiral_72/Spirals_page.html