The tanks we have built for the local zoos and museums here have sealed tops, with one access panel that is clamped down. All of the filtration is from a sump located below the tank, attached with sealed in/out piping. While this is somewhat $$$blingbling, it does work quite well...some octos don't ever seem to want to crawl out of tanks, and some are little houdinis...guess it depends on the personality!
Greg