Species vary considerably with their tendency to escape. Some never venture above the water line, others don't seem to pay any attention to the air-water interface and climb up the walls with no hesitation. This probably has something to do with the habitat in which they are usually found, size, etc. but I've found it almost impossible to predict who will stay at home and who will go walkabout. In the end, you pretty much have to assume that any species may try to escape.
Since we keep a lot of venomous octopus, I have to be particularly careful to prevent animals from wandering down the hall. The one system that has worked without an escapee is to custom build all glass tanks with a smooth, even edge all around the top. A heavy glass plate forms the lid. We either tape it in place or place a heavy weight on the top. Octopus can squeeze through very small cracks, so there can be no gaps. One slight modification that works is put strips of weatherstripping foam tape on the glass lid where it meets the walls. Again, no gaps allowed, but octopus in general do not go through foam and this creates a good seal.
Filtration is provided through canister filters that are plumbed through holes drilled in the top corner of the tank well above the water line. Depending on the size of the tank, you can either use bulkhead fittings or, for smaller tanks, we use a two inch piece of flexible tubing just slightly larger in diameter than the hole Hose connectors are then pushed in from either side to make a tight seal. We don't use extra lighting, so I don't usually worry about ventilation, but when necessary, a third small hole can be fitted with an air line. I would not recommend this system for use with bright, warm lamps. The last thing necessary is to prevent the octopus from entering the water input and output lines. For the input, we use a spray arm with many very small holes mounted above the water line. For the out, we fit a piece of foam over a strainer on the tube. As long as the foam is tightly in place, we have never had and animal go through it.
I've tried alternative methods such as Astroturf, Velcro, mesh, etc. and they all fail occasionally. The sealed cube approach works best for us. We currently have about 25 such systems running and we haven't lost an octopus (or person) in years.
Roy
