While I have no cephalopod experience, even the average visitor would be able to determine that the tank was too small. When fully stretched out, the animal could not fit depth-wise and as for length, it would stretch nearly 3/4 of the tank.
For the sake of argument, it appears you are presuming that the animal's environmental needs for space meet your perceptions of spacial needs. I am not saying that I am an expert, so please don't mistake that. However, could it possibly be that this animal prefers living it's life wedged in small crevices in rocks and therefore might be stressed out by expansive space. I'm not saying this is so, I'm just challenging your opinion and proposing that things may not always be as they appear.
I think the animal was stressed at was- when I took the photo, the little kids had finally gone away, so this was the only time I saw it settled. This exhibit provided no den space, so even though the octopus was walled in on all but one space, there was nowhere for it to hide. Most places with no den space for octopuses see little activity and just have octopuses as tank decorations. The tanks downstairs in the special exhibition, for common and day octopus, did a great job. Both provided den space and I saw them using the space and also being active. That's all speculation, but it makes sense to me. This poor guy, however, barely had room to go anywhere. Yes, octopuses are seclusion species, but they also like some areas to swim.
