The specis that are currently available though octopets, fishsupply, live aquaria... Sepia.O can not be kept in any thing less than a 180 gallon when they are full grown due to their adult size and waste put out. Some of the smaller species like Sepia. bandensis can be kept in smaller aquariums but they are very hard to come by in the trade. Most of the bandensis that come into the county seldom make it or live for more than a few weeks. If your looking for something to but in a 20H I would try a pygmy octopus like O.mercatoris. teyt can be collected in the winter months by places like tampa bay saltwater.
Another thing you might want to look in to that would be fun to keep in a 20 gallon is a nice mantis shrimp. They are highly intelligent and very cool to watch. The mantis forum has lots of good info on this. I have kept several different species in smaller tanks and I must admit I have become very fond of them.
chris