A 100 gallon canister seems good for an octopus. There is no such thing as too much filtration with an octopus. I remember one person had a 110 gallon rated filter on a 10 gallon tank.Mercs like a temperature of 79 degrees, +/- 1 degree.
As for food, live food will be a must for the first month or more, as it will no doubt be wild caught. You can buy mysid shrimp and shore shrimp from Sachs Systems Aquaculure, here:
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All cephs eat a lot, but I cannot find any definite value on how much mercs eat. Amin for 3-4 mantle-sized foods each day. I would get a picture of the octopus you are purchasing so you can get the appropriate food. You will need a seperate tank for the ffeder, with a filter 2x the size of the tank as you will have a lot of animals in it at once. According to Cephalopods: Octopus and Cuttlefish for the Home Aquarium, a 15 gallon tank can support 100 shore shrimps, or about 250 mysid shrimp.
Also, if you live near the beach, get some small crabs.
In your original post, you said you wanted one that will interact with you. In that case, I would get a 50 or even an 80 gallon aquarium and purchase an abdophus aculeatus or an octopus briareus. AA for the 50 and the OB for the 80.
Bimacs are also cool in a 50 gallon, but you can only get them captive bred since catching them is illegal now.