What everyone else said. Giant Vals, or "onion plants" or "jungle Vals" (all same plant), commonly available in most LFSs.
They grow at least 3 feet tall and are like weeds. They reproduce quickly too. A lot of people in the heavily planted tank side of aquarium keeping don't really like them because they grow too fast, need trimming every few days, and suck up all the nutrients and C02 from slower growing plants.
In your case, they sound perfect.
I used to have a bunch of them in my old 50G setup. I started out with just 2 bundles, and had enough to fill in 4' of background within 6 months or so.
Here's a picture of them at their prime. Its a 22" high tank, and they're growing about a foot past the water height. I had to trim them back to the water height every week, and after just 7 days it would look like this again. BTW, nothing special in this tank for plants. It had high lighting (128W florescent tubes), but no special gravel, fertilizers, C02, etc.