to TONMO.
Goldfish are very much not recommended: any freshwater animal isn't a healthy food in the long run. Almost all octos will eat live crabs and shrimp, but only some will eat frozen foods. Some will also eat clams, scallops, and snails. We don't recommend feeding fish at all, although sometimes they will eat them, partially because it's not the proper diet for most species of octo, but also because many fish are treated with copper-based ich medications that are lethal to octos in very small amounts.
We recommend tanks around 55-75gal for bimacs or briareus, so a tank in that range is good. People have tank and bucket sumps, I don't think there's much preference, although reading up on octopus escape-proofing requirements is important. For a bimac, it's ideal to have a chiller, but not absolutely required. Lighting requirements are minimal, most octos find it stressful to be in bright light. They also produce a lot more waste than most things in a tank of the same size, so it's important to have extra filtration, a skimmer, and a well-cycled tank, particularly since they're also very sensitive to water quality problems.
Larger octos are usually more interesting, longer-lived, and easier to keep than the dwarf species, so ideally, a tank-raised bimac or briareus, or a young hummelincki or aculeatus would be the best bet for a first octopus. Vulgaris gets much larger, so it's impractical for most home tanks, and dwarf species like mercatoris tend to be shy, nocturnal, and short-lived.