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Well, you are going to definitely need some way of removing waste products beyond that...a power filter or a wet dry system could be used, depending on your preference. Octopus put out a tremendous amount of waste, several times that of a like sized fish!
So in a 75 gallon tank with a single bimac I will need:
75 LBS LR - Generallyoctos are not filtered with live rock alone like reef tanks. Live rock can filter small fish and inverts well, but I feel that even 150 lbs in a 75 gallon would need alot of prep (slowly building the ammonia load) to finally be able to handle the large amounts of waste from a fast growing ceph. Since live rock is totally submerged it is not as efficient in converting ammonia as a wet/dry.
A protein skimmer rated for 150 gallons - Since they make so much ammonia, tons of nitrate will soon follow. Skimmers also remove ink from the water and are considered essential for octo keeping.
A Refugium - This is really for additional nitrate removal via nutrient export in macro algae.
And a wet/dry? - The real workhorse in the filtration system.