Octopus

It's not a good idea to keep crabs in the aquarium with an octopus. They do rest/sleep and in the confinement of the aquarium the crabs can pinch the octopus.

The smaller octopi that I've seen available get to be 14"-16" whereas common octopi can get to 2'-3'.

I had a common octopus many years ago. She had mated in the wild and evidently the conditions in my tank were suitable. They mate and then when conditions are right they break the sperm packet the male deposits and fertilize the eggs. She attached her eggs to the roof of her cave. It was amazing to watch the baby octopi develop and eventually hatch out. They were about the size of a dime from tentacle tip to tip. Unfortunately, the brine shrimp eggs I had did hatch in time and the mortality rate was very high.

Not that fast, but depends on the species.

make sure you get a very tight fitting top, they are highly dextrous. I beleive there was a thread about a guy who kept one in a pico a while back(there is a species which stays small, still wasnt a great home for it, but whatever)
anyway, the octopus crawled out of the tank and wandered his table before returning to his tank.

Good luck with your endeavor!

most octupus grow to 1ft+, and a a 20g or 55g isnt going to cut it. In the occurance that the octupus would release its "ink", it would definitely kill itself.

IMO, i'd stay away from them.

a herd of crabs wont last more than a week. lol. the octopus would eat them all. And yes, a species isnt listed form that website. There are two common types of octopi in the aquarium trade. There is octopus bimactus (bimac octo) which dont get very large. And then it could be octopus vulgarus (the common octopus) which as listed above, get very large.
i woudl order one form a place that has aa listed species. also, keep in mind they dont always ship well and you cannot keep ANYTHING else with them. I've actualy seen a large peacock mantis shrimp absolutly destroy a small blue ring octopus... but that is besides the point...
they are also very messy eaters, i would suggest the 55 and get a good skimmer if you can and as suggested, make SURE to seal EVERYTHING off. also make it hard for anythign to be lifted up. you can put some astro turn aroudn the edges of the tank out of sight. they dont like the feel of it and will often discontinue their escape extravaganzas if it is in their way to an opening. lol
keep us updated! they are by far my favorite animal.

Let me emphasize that octopi are NOT for novice aquarists. They require impeccable husbandry and will die otherwise. There are invertebrates and are subject to the same toxin vulnerability that others do.

If you put a border of astro turf on a frame around the top, you can have an open top. The border needs to be at least 4-6" tall and must fit snugly around the top of the aquarium.

Octopi like very subdued like. Most are nocturnal hunters, so don't expect to see them out crawling around the aquarium. They spend most of their time hiding. The one I had was an exception and spent a lot of time out in the open, but I haven't seen another one since like her.

Pretty much no matter what size closed system an octopus is in, if it inks, it's pretty much going to die unless there is immediate intervention. Mine was in a 30 show, but it required daily maintenance.

Feeding is hit or miss. Some will take dead shrimp while others will only take live shrimp or crabs. They will eat fish, but don't even think about feeding them feeder goldish.