Depends on how far you get into it to be honest. Getting the tank is the cheapest part. People have been successful keeping an octopus, but they are escape artist, so you cannot have ANY opening on your tank...Not even small ones. Also, you cannot have much flow at all due to them getting blown around or sucked into a powerhead. Cool creature, but not much you can add with them that they won't eat. IMO, they would make for a pretty boring tank.