I have what looks like a grass-type plant in my tank. My fish don't eat it, but they swim through it alot. Anyway, I just went in to switch the light on and noticed that the plant has grown roots and now there's four little plants growing around it! I didn't think this was possible with gravel, I would've thought you needed dirt!
I'll need to do a little underwater gardening before it takes over my whole tank!
Thought I'd share that, cos it made me chuckle, hehe.