D, I had to put this here as well it was a great post! I'm not going to put it in quote bubble but DWhatley wrote this for another thread and I copied it to here for future reference.
My List of Successful Occupants
* Brittle stars - great choice, great cleaners. I keep one red one in each of my octo tanks. Both these and serpents can be hand fed if you feed daily at the same time.
* Serpent Stars - another personal favorite housed with my octos. Avoid greens as they will become aggressive once they are a few year old, personal verification
* Caribbean Common Starfish - not a problem with octos but even large tanks have a hard time supporting them food wise (anticdotal, they my just get old and shrink after several years as I can't seem to verify the natural lifespan).
* Most Gorgonians - They need high flow areas. IME these do well in the larger tanks (50+ gallons) but not as well in the smaller.
* Mushrooms - Getting them to attach is the only problem, the red ones from the Caribbean seem to be octo-proof and have stayed in place even when senescent animals have worked at trying to remove them.
* Leathers, Xenia, Kenya
* Feather Dusters - place where they won't be molested.
* Low Stinging Polyps - It is hard to know the impact of the polyps, if you add polyps observe the octopus. If it touches the polyps and reacts by quickly removing the arm, take out the cluster before it spreads.
* Common Clams from the seafood market - These are not pretty, might help filter your bottom substrate, are hardy and may ocassionaly go missing - as in eaten. They are a lot of work for the octopuses and when octos know there is easier food, mine have ignored them. If eaten, they are far less messy than oysters or muscles and I have never had one detectably pollute a tank. Forget any kind of attractive clam, it will not survive.
* Peppermint Shrimp - I have found that starting with a pair before there is an octo in the tank and then introducing an octopus usually lets me have one shrimp over several octopuses. Anecdotally, it appears one will observe the other being eaten and then learn to avoid the octopus. The reason one survives and the other is eaten is conjecture but I have experienced it more than once. Peps are not particularly attractive - almost creepy - but will help keep some forms of small aiptasia from heavily populating a tank. They don't seem to like the larger ones and some varieties but for the ones they will eat, they do a great job. I don't recommend keeping more than two in a tank because they can pack and become aggressive. I have seen them try to pick on an octo but that is usually short lived and only during acclimation or when an octopus is dying.
* Snails or Hermits - Add a handful or more at a time. Some will become food but they make a decent clean up crew. Typically, I will keep one or the other but often keeping both eliminates the snails. I often wonder if the reports of octopuses eating snails is actually consumption by hermits.
* Pencil Urchins - Good rock cleaners but they will munch on some forms of soft coral if there is not enough algae.
List(animals to watch because of water quality but won't harm the octopus):
* Cowrie - (I keep one in one of the 65's and am very fond of it. Joe-Ceph reported a major problem with one sliming and I did have some slime the first year I had this one but no problems at all since and he has been kept with numerous octopuses.
* Cucumber - I have not kept a cuc in with my octos but many have. Keep away from the exotics as many are responsible for the infamous cuc nuke syndrome but the commons are harty and don't seem to pollute like the exotics even when they do die. I recently had one to find a power head intake in the sump or our nano - obviously he should not have been IN the sump - and mutilate the animal but the tank did not suffer. There is at least one from the Caribbean that don't bury and are enjoyable to watch.
* Low Stinging Anemones - I hesitate to add this one and don't keep them with mine but a couple seem to be tollerated in a larger tank. Typically these will sting if contact is made and stings can damage the skin, setting up a potential infection.
* Ricordea - these do have a slight sting and should be used sparingly and in places the octo would not typically travel. I have only kept one ric and one flower with any of my octos. The small ric does not seem to be a problem but I have seen Diego react strongly to the flower (it was in the tan before he came and removing it would mean killing it so I am monitoring).
* Pincushion Urchins - There are warnings against using this kind of urchin but I have seen many people keep them in an octo tank without a problem and suspect the warnings are unfounded. However, I do not keep them with my octopuses
most anything not listed above but especially:
* Any kind of fish. Sooner or later one will be a problem for the other.
* Seahorses. Guaranteed meal, they have no defenses and even if not actively attacked will ultimately be touched and then consumed.
* Long Spined or Rock Urchins - the spines on these break and embed easily and can be another source of unnecessary infection risk.
* High Stinging Corals - the list includes most hard corals and anemones.
* Aggressive Sea/Serpent Stars - Green brittle star and any meat eating star that can trap its prey. Anticdotally, we may have lost one octo to a Bahama Star that trapped and killed a sleeping octo. It is possible that the apparently healthy animal died and the star was scavenging but I keep it on my no list because of its behavior.