as someone who’s kept octopus a number of times before i think the most important thing for an octopus tank is definitely having a lid to prevent it from escaping. second is the filtration. system could use a skimmer as they produce quite a lot of waste. for tankmates avoid fish of any kind, they will likely either harass the octopus or get eaten. inverts like starfish should be fine as clean up crew(crustaceans like shrimp & hermits will prolly become food). the rest of the setup should be similar to a typical fowlr.
1) similar to fowlr, personally i’d go with at least 10x tank turnover return flow. having rock structures big enough for it to hide in/amongst is ideal
2) definitely something more subdued, most cephalopods are sensitive to bright light and octopus definitely feel more comfortable coming out when it’s dim
3) mechanical filtration like socks/pads/floss + protein skimmer suitable for your system volume(also oxygenates the water a lot)
4) weighted lid cover, can try redsea’s customizable product for this
5) might wanna run some activated carbon in a reactor or a mesh bag in the beginning to double as water filtration along with the skimmer for any ink the octopus might secrete
Hope this helps
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