Open top tanks always make me nervous. I have eggcrate over my clown tank as it's very shallow (12") .. the clowns basically hang out in their anemone all the time but I've noticed in particular my female, as she gets more comfortable with her surroundings, tends to wander further and further out to patrol or expand her territory I suppose. In the 90g before I took it down she would go to the farthest corner of the tank (whereas in the beginning never more than 3" away from the anemone home base). Same thing with this new tank, at first she wouldn't leave the anemone whatsoever, now at about 3 or 4 weeks into the new tank she's bold enough to come out about 12". I hate the look of the eggcrate over the top though, it looks so ugly, so I'm thinking maybe some cover glass. I'll have to figure something out. For now it's the eggcrate with a little cutout "flap" attached with zap straps for easy access for feeding.
A few years ago I had another pair, had a tank disaster in one so I moved my fish over (a yellow tang and the clown pair) into a 20g I had going. After a few weeks I decided the fixed up tank was cycled enough to move the fish back, so in the morning I went downstairs to start the catching process, and on the stairway I froze: I saw an orange lump on the floor. It was the female, completely dessicated. Of course she was dead.
It irks me to this day that I had I made the decision one day sooner to move them that she would not have died. So yeah the moral of my tale is: clownfish can jump just as any other fish, take precautions to prevent the carpet jerky fate.