Hi, Have had salt water tanks on and off for a few years and had an Occy for a short time when I was a teenager. My kids are teenagers now and have custom built a tank for them. My daughter caught a O. tetricus (Gloomy Octopus, common to the Australian east coast) which has been in the tank for 2 days. Will try to feed him, her, it tonight (have been trying to look). Each leg is just over 30cm or 1 foot, giving the the Occy of over a 2 foot span. Mature they can grow up to 2 meters or 6 feet plus.
Happy to chat.