I keep several species of pygmy and dwarf octopuses. After fighting the containment battle for several years (velcro, duct tape, etc.), I finally came up with a tank that has yet to fail. I make my own all glass tanks, but a 10 gal with a tight fitting glass lid works well. I use a canister filter and drill two holes an inch from the top of the tank. The diameter of the holes should be the size of the tubing you use. Take a barbed 90 degree hose coupler that is a tight fit for the hose and push an inch and a half length of hose onto one end. Push the short length of tube through one of the holes that you drilled and then push the second coupler into the other end of the short piece of hose. If the hose is the same diameter as the hole, as you push the couplers into the short tube, it will expand and tightly seal the hole. Repeat for the second hole and you now have u-shaped connectors into and out of the tank. Attach the intake and output hoses from the canister filter along with a spray bar and a water intake filter and you are ready to go. To keep the octous from crawling into the intake filter, I like to use a piece of sponge pushed over it, but some type of filer screen material secured with a rubber band also works. If you need more ventilation, you can cut or drill a hole in the lid and glue some window screen over it.
Roy