I read through the forum thread and was pleasantly surprised by much of it. Often self styled experts show up on these topics and make conjecture without supporting experience or direct knowledge. I will quote and address (based upon my experience) the issues brought up by the one that pontificated:
For starters: Absolute minimum required size tank is 120 gal, larger is better.
FALSE with qualification. This is only true of the larger animals, O. vulgaris and O. maya (we don't see O. maya but it is similar to O. vulgaris and is being raised as food in Mexico, I am hoping some may show in the pet trade). We do recommend starting with a minimum of 55 gallon tank with sump and prefer to see 65 - 75 gallon main tank. There is only one commonly kept and not dangerous, O. mercatoris, that is just fine in the size you currently have and it would be lost in anything much larger.
And they are extremely strong and all rock needs to be glued solidly down lest the octo pull it over onto itself and die.
FALSE You do want to arrange the rocks in a sturdy fashion and larger animals (I have never had a merc to move a rock) will often move them around if the arrangement is not to their liking
and when they are about to brood. Death by rock rearrangement has never been recorded on TONMO.
You cannot keep any other creatures in the tank with it that you do not intend for it to eat, because it will.
TRUE Generally speaking this is true, like many animals, a species only tank is
required. There are a few animals that can be added for interest and as clean-up crew.
Most importantly you cannot have it in such a setting where it might get startled, such as by you walking or moving too closely in its vicinity. This may cause it to release its ink cloud in panic. This will require an immediate almost 100 percent water change or the octopus will die.
FALSE Octopuses DO need several dens where they can FULLY hide and be comfortable (don't skimp on the Live Rock so you will be able to see your animal more, it will have the opposite effect) but normal day to day traffic is not a problem and is more likely to make the animal more active/interactive over time. We keep ours in the eating area where they see us at regular daily times. They see us calmly eating but they also see the dogs running around and a lot of through traffic as it is the access to the garage and a central traffic point in the house. We highly recommend a skimmer to handle inking. In general inking is small and rare and a skimmer will handle it. We do recommend a NORMAL water change (2-5 gallons) during an inking event but unless it is very heavy (rare) the skimmer will remove most of it before you can change the water. Ink is interesting. There are basically two kinds, a thin and a thick. The thick (used for pseudo morphs) can often be removed with a fine shrimp net or turkey baster. What inking we do see is usually done in the first month or two. Inking in a shipping bag, however, is almost always fatal.
It requires dim light at most, meaning that you are not likely going to see much of it.
TRUE and FALSE. Octopuses would be happy with ambient light only but normal T lighting (halide is too strong, compact fluorescent common but should not be reef intensive). There are noctural (night active), diuranal(day active) and crepuscular(sunrise and sunset active) octopuses. With the nocturnals, we recommend adding a red light at night to allow you to watch the animal and still give it a night quality to the lighting (I usually have a red light, inaddition to my normal tank lighting on the crepuscular animals as well). How often you will see the animal is very dependent on the individual animal, its natural active time, its age and the species. Diurnal animals do need a dark period and become nervous if you don't provide one (I call it sleep, there are technical arguments/questions on the use of the term but dark down time is necessary)
The mention of double-hulling the tank to keep octo and heater totally away from each other was noted above. It is highly recommended to keep the heater (if needed, often it is not - if the tank stays a consistent temperature between 75-78 for a tropical species, no heater is needed) where the octopus can't touch it. This is another good argument for a sump.
Add on the extremely short lifespan as Arakkis notes and you are talking about a monumental expense and effort, for a very low enjoyment factor, for a very short time.
TRUE/? lifespan is short but exenses to keep them are not monumental (I have found the dwarf more expensive to feed than the large ones though). Most octopuses will eat thawed table shrimp and most any crab. Mussels are messy so mine don't get them often but I do keep live clams (the kind you buy at the seafood counter on ice) in most of the tanks that are occasionally eaten. The larger ones can be fed blue crab claws and we get them from our international markets by picking through the live crab bins and then freezing them (whole crabs don't freeze well and can pollute the meat). Occasional crawfish and live fiddler crabs round out the menu. Hatchings, on the other hand, ARE very expensive to try to feed and chances are not good for their survival. Low enjoyment is in the eye of the keeper of course, I will say that they require patience and persistance to have the highest levels of interaction.
I understand the fascination with octopi and share it. The first time I saw one of these dwarf/mini types I lusted to have it too. But I just have to urge you to withstand this temptation. I would hate to see these become some kind of a fad in the hobby. Above and beyond our own personal issues the last thing we need to be doing is puting any more stress on the populations of any sea creatures, or any wild creatures for that matter.
We do worry about the cephalopods that are losing habitat (see the exotics forum) or are dangerous and do not support their keeping. The ones we do keep are not endangered and some are harvested for food. If we do see captive breeding, it is more likely to come from the food industry as a side line than from the aquarium trade out of practicality.
If you've ever seen the figures on how many birds, for instance, die in capture or shipping to fulfil the demand in the hobby market you know how devastating to populations it can be. (Yes there are captive breeding programs but they never seem to make a dent in the wild-capture market.)
I can't address this directly, there are definitely shipping losses and octopuses must be shipped overnight with a lot of oxygen and even then we lose some. I can suggest that you look at animals coming from crab traps (by catch of the crabbing industry) if you are concerned with reducing wild populations. The aquarium trade that offers these animals acquires them from crabbers that would otherwise use them as bait (octopuses eat their main catch). Any extended life beyond the crab pot is more than it would have outside the aquarium trade. Most animals are not collected this way but you will find some in South FL.
The last brings to mind another point worth mentioning to new keepers. Wild caught octopuses will not have all their arms. Those same crabs do fight back or win a battle or two. The arms will grow back but often new keepers confuse the natural loss with collection methods.