Small gaps underneath rocks on bottom?

Hey all... Could use some help. I recently lost a dispar due to it swimming to quickly into a narrow opening at the bottom of my rocks. It basically got itself stuck and by the time I found it... It was too late. I really want to avoid this happening again. Mind you, a few bottom gaps have multiple openings... These are fine. However, a couple are small and only have one way in and out. I was thinking of pushing some sand into these areas to prevent another accidental fishy suicide. Thoughts? OK idea? Bad idea?