Ok yep, that's probably what it is then. I had one in my 20 long, and I didn't realize that it could get huge if I didn't prune it. It ended up taking over the tank and I threw most of the plant away... but now i have TWO new lilies that are growing from those roots :P They are tiny though so this time I'll watch them closely to make sure I remove the leaves before they get too huge.
They are gorgeous plants though, and they do well in lower light. You just push the bulb halfway into your substrate, and once it sprouts push it all the way in. I'd say only about 1 in 4 of the bulbs I bought sprouted at all, so make sure you buy the kind with a replacement guarantee that you can mail away for. I think the ones in the pet store sit around a little too long.