Based on stories heard and what we've read from the board, I'd say it's easily worth the trouble to keep an octopus and we'd try again if necessary. It's just that we spend so much time stressing about this one..
I'll recount my experience from the acclimation about what we saw of it.
It was about 2-3 inches, and spent most of its time curled into as tight a ball as possible in the corner of its bag. It was maintaining a uniform light gray color and had a lighter, almost white underside. The mantle was short and we did not notice it to be distended or floppy.
Before transferring, we introduced a small fiddler into its acclimation bucket to make sure it ate before going into the tank. It stayed curled up, but used a single tentacle to push the crab away if it got too close. We never could get it out of the bag and had to transfer it to tank with that scrap of plastic. For this reason, we never got photos of it spread out or moving. Below is the only shot I have from then where you can somewhat make it out.
Rocktopus has just been spotted outside of her den breathing VERY heavily and not moving her tentacles. The mantle seemed indented almost. She didn't make any attempts to remain right-side-up and to my untrained eye seemed to be displaying symptoms of senescence . We have separated her from the hermits in the tank. No eggs visible in the den, but its too deep and twisty to really tell.
Should we do anything else for her at this point?
If this is the end for her, we are determined to try again. What should we check about the tank to make sure it's hospitable to a new octo and there's no risk this might happen again?
You can do everything right and it still might do the same.
The older the tank, the better I have noticed. Lots of proper tank mates and clean team (potential food).
I learned if I can keep shrimp alive, it's a good indication of the levels in the tank. Too soon, will just kill things. It's painful to wait for a tank to mature.
Buy fake plants or places to hide or hermits to help fill the void. Hermits are funny to watch.
Resist getting a star, especially sand sifters, they die. Pistol shrimp are fun. They eventually get eaten, but they dig, make homes, move sand, and click until they get eaten.
Pencil urchin and brittle stars are cool too. Try to get cleaner shrimp in pairs, but not too many, they can pick on your ceph.
Giving it places to hide helps. Maybe give it glass shrimp or much more food to hunt.
If you have a kritter keeper or breeder net, you can put Rocktopus in there.
She may have already laid her eggs before being introduced to your tank. Since it lived this long, acclimation does not seem to be the problem but be sure to allow 3 hours of slow water exchange when acquiring one that has been shipped or the LFS water is different from the water in your aquarium. The PH of shipped animals is almost always significantly different.
I suspect you received a senescent animal but when you mention tank parameters, please give the actual numbers for salinity and PH as there is a recommended difference for salinity (lower for fish than for corals and inverts). Also confirm 0 nitrites and ammonia. Either of these not being zero can cause senescent like symptoms and death. Nitrates are less I important for an octopus (not so for many corals).
I think @Jocco meant, avoid star fish but consider serpent/brittle stars (correct me if I misinterpreted, Jocco)
We did our last water change just more than a week ago, so I was expecting ordinary conditions, but I just performed another test and here are the specs:
Salinity has been maintained at SG of 1.026
pH was and is around 8.2
Ammonia is between 0 and 0.25 ppm, closer to 0
Nitrates have apparently risen recently and dramatically, to 10ppm.
Nitrites too have spiked to 1.0ppm
This is bad. Obviously we will need to perform a major water change before taking any other steps, but what could've caused such conditions to occur? How can we increase the level of our biofiltration?
The ammonia and nitrites are a definite concern (nitrates are fine). While you have an animal in residence, there is no good way to continue to cycle (increase your biofiltration) the tank so frequent water changes are the best way I know of to minimize the negative effects of decaying food and waste. When you lose this one, you can increase your cleanup crew and then over-feed them (I recommend ground grocery store shrimp in small quantities but meaty dry foods will also work). Hermit crabs, serpent or brittle stars (avoiding the green brittles) and pencil urchins will all eat the meaty scraps and can remain in the tank with an octopus (hemits may become supper). This method WILL raise your nitrates (and you still want to do regular water changes). Snails are always welcomed for algae but won't help with cycling.