Use pothos. Cant stress this enough. The roots look amazing hanging into your tank, and the ability of a large clump to remove nitrates is mind blowing. I have a large clump of pothos on top of all of my tanks and have never had an issue with nitrates.
Just pick up a few pots at lowes, a local plant supply store, or home depot (Wal-mart is pricy in my area). After you procure it, hose the roots off as thoroughly as is possible. Do the same with the actual leaves. Do NOT allow any of the little white beads or dirt to get into your tank. The beads will spike your ammonia to unimaginable levels. I let mine soak in the kitchen sink for a few days after a thorough hosing. Never have issues there.