Is it possible your fish are uprooting them? My rhom tore up every plant I tried to put in his corner, at first I kept replanting it thinking that they were just floating up but when I came home and found it torn into 6 or 7 pieces I knew it was the fish doing it. It's possible your fish just don't like your plants.
Failing that, if they are just floating up and having trouble staying rooted you can buy plant weights from the lfs that should hold them down.