Rocks work, just use superglue or thread to tie them on--it's what I did with some of my anubias. It's funny...I was just looking at my tank after this thread and my java ferns are sprouting babies like crazy. One had already come off of its parent leaf and had floated up to hang out with the hornwort. I knew something was there a week and a half or so ago, but until yesterday it hadn't really grown enough leaves to show what it was, but sure enough, it's a baby java fern. hehe