Hope you have a good tight lid to keep the escape artist in the tank.
Also a skimmer would be a good idea. Since octopus don't live very long in captivity(or in the ocean for that matter.), it's probably going to foul the water up in a 30 real quick when it croaks. A skimmer might help keep the water clean long enough to get the octopus out of there when the time comes.
What kind of octopus are you planning on getting? Venomous?