How do you keep worms out of swimming pool?

Pile a small line of crushed shells along the pool deck edge. The sensitive worm's tummies will prevent them from crossing over to the pool. The problem is the worms crawl out during rain, then when the rains stop, the worms dry out and head for the nearest source of water - your pool. Stop them from entering your pool deck area in the first place with crushed shells. Ian D.