I'm glad to hear that you're doing your homework and preparation to keep an octopus before you get one. Have you ever kept a marine aquarium before? If not, you've got two areas to educate yourself in: octopus keeping, and general aquarium keeping. I suggest that you first read all the articles (not just forum posts) here on Tonmo. Then go learn about how to keep a marine aquarium. You especially want to understand the "nitrogen cycle" which governs how your tank processes animal waste into less toxic substances. You should find several articles online about the nitrogen cycle, and try to hear it described in different words, so that collectively, they will cause you to have more complete understanding. Be careful not to trust any single person's opinion about anything, but instead try to find out what the most common opinion on each topic is ("trust but verify" - the Gipper)
Be patient, and thorough, and try to think of every relevant question, and get it answered, before you take any action. That will save you the time and money of needing to backtrack after going down the wrong road.
go read, and then come back with questions to fill in the blanks.