Depends on the species. Most gonodactylids do not jump very often, although some can clear a couple of inches. The one group where a cover is for small odontodactylids such as O. havanenisis, O. brevirostris and O. latirostris. Six cm animals can (and do) easily jump 10-15 cm. Even a large O. scyllarus can clear a few cm. Perhaps most surprising, last year I had a 11 inch Hemisquilla jump out of a tank with about 3 inches of freeboard.
Roy