I had other herbivores in the tank like Annostomus but when I tried Vallisneria even the giant types they got nibbled off at the base and I got floating plants. Your best bets are tough leaves epiphytes like Java Fern, Anubias and Bucephalandra or Amazon Swords could be a safe bet, fast growing stem plants might be an option I got a real jungle of Hygrophilla Siamensis 53b at one point.What if vallisneria was there when it moved in?