And I had that thing in a 10g tank too... :affraid: It looks fantastic in your tank, Poke. :spoton: I'm glad it's doing so well for you.
Amazon swords are pretty undemanding. If you abuse them they'll just grow slower and uglier, but they're nearly indestructible. And if they're even marginally pleased with themselves they send out runners everywhere that will eventually fill up the tank, given enough time. LJ is definitely right about keeping them trimmed.
They do seem to need a lot of potassium - I get lots of holes in their leaves in some tanks without root tabs or dirt. They'll grow in just about any substrate, but as LJ said, nutrients are necessary to keep their leaves green and hole-free.
They send out a massive array of roots - it's lots of fun pulling them up. I would recommend them for tanks of 29g or larger - they need at least 18" of tank height if grown in ideal conditions. 20s are a bit short... I still have them in 2 of my 20s.
Anything "medium" lighting or higher will do. I grow them very well with 2x 13w 6500K CFLs in my 10g tanks and 2x 31W T5HO bulbs over my 20g tanks.