Do you still have the little weighted ties on them that were around the plant stems (if they had any) when you bought them? If the bases of your stems are bundled together with those things, even if the fish dig the gravel away from the plants the weights will still hold them in place and keep them from floating. After a short while, they'll begin to develop root systems and will pretty much hold themselves down.
I didn't like the weighted ties and took them off my plants at first, but after a couple of weeks of exactly what you're describing, I put them back around the stems just below gravel-level. No problems since, but I don't have any of the particularly persistent plant-diggers/destroyers either. Yo-yo loaches are the worst culprits I have, and although they dig at them a little now and then, they aren't too bad.
pendragon!