I bought some staurogyne repens at petsmart the other day, divided it up into smaller plants, and stuck the plants in my gravel. when I woke up the next day, they were all floating at the top of the tank, so I replanted them. They keep on floating up, and will continue to do so until they grow roots (which they don't really have yet). How can I keep them in the gravel? I do not think my fish did this by the way, as I have only seen shrimp in there until now. I do have some floating hornwort, which the filter seems to be sending underwater. I think the hornwort then hits the plants.