Silver dollars and plants just don't go together, as you are discovering. I really wanted a school of silver dollars as the main inhabitants of my 55, but can't do it because of the plants.
There are some extremely natural looking silk plants that you can consider, or build a cork background, to which you can attach java fern, if it seems they do not have a taste for it. It may be that after they are done with the other more delectable plants they will move on to the java fern, but you can give them a big ole romaine lettuce leaf every couple of days and that might distract them.