a lot of stem plants actually do pretty well as floaters. Now they won't last long tied to a rock, that part will deteriorate and you'll have to keep trimming it and re-tying it down. I'm doing something similar right now, but in a nano so I can keep the stem to like 4 inches tall. Plus I have a rock crack to cram it into so I don't have to tie it. It looks like it's "growing" from the substrate but it never really roots.