Definitely not a good starter critter. They are very cool, and easy to keep. We've had 2 octopusses (which is the correct plural--it's a Greek base, instead of Latin, which would be octopods or octopi). The first lasted about 8 months, the second we had for about one year. The problem is that most octopus available for sale are females. As soon as the female finds a happy place, or can build a cave, she will lay eggs. She won't eat--or even willingly leave the cave--until the eggs hatch. This means she starves to death, and the eggs usually are not fertile (50-50--the first ones were, but we couldn't feed the fry, the second ones were not fertile).
Our setup was pretty basic--we plumbed a 29 into our 65, so they shared the same filtration and water. Each tank had an outflow, and one return into the 65. The outflow from the 29 went through a canister filter, just in case we ever needed carbon (in case of inking, basically). An overflow, covered with nylon, between the two tanks put the new water back into the 29. Then we had a tight fitting glass lid, which was weighted down as well. We fed a variety of foods--small chunks of fish, an occassional feeder, ghost shrimp, crawdads with claws removed, etc. She ate any crustacean that hid away on the LR--crabs and such. Snails were ignored. Of course, until the eggs--then nothing could entice feeding. Just used NO lights, on a timer--they don't like bright lights.
Bottom line, I wouldn't get another unless we could be gauranteed of getting a male. Both of ours were very entertaining, the second was very sociable. They are smart, and cool, but it's heart wrenching to watch them waste away in a futile effort to reproduce.
I dont know anything about saltwater tanks, or the keeping of octopus, or the sizes, etc.
