serpent and brittle stars are great tank mates and there is quite a selection if you go looking. They can be trained to take food from your hand if you care to work with them and on occassion some will have a bit of personality. Avoid the green ones though, as eventually, they
will become aggressive. The only starfish (sea star
) that has been reported as a possible problem is the Bahama star. The incident is anticdotal but worth noting. They are aggressive feeders and a young hummelincki was found trapped in a shell and being consumed by the Bahama. It is unknown if the apparantly heathly animal has simply died and the star was cleaning up or if the star was the cause of death. Starfish in general are difficult to feed and some difficult to acclimate. This does not hold true of the serpents and brittles though. I would avoid the sand sifters as very few survive more than a couple of months. I have several common Caribbean stars that have lived a number of years but they have shrunk in size and likely don't get enough to eat when there is little algae about (the serpents and brittles are meat eaters and live for years on the leftovers). I keep roughly one per 25/30 gallons of water in an octo tank but we do feed the tank additional meaty foods to be sure the serpents get enough to eat.
Be careful with urchins and stay with ones that do not have sharp or easily broken spines. Pencils do well and I have them in most of my octo tanks (they are not reef safe and will eat soft corals and macro algaes if they don't find enough algae on the rocks). I don't keep pin cushions in with the octos but several people have/do and no one has reported a problem. Avoid the rock and long spined animals entirely (for an octo tank). I use roughly the same rule of thumb for the urchins as the serpents but will move them from tank to tank if I have the beginnings of an algae problem on the rock as they do a good job keeping it mowed. Do note that the pencils will happily consume coraline algae and I have used them to reduce it when my tanks have looked entirely purple.
Another good critter to keep is a peppermint shrimp (no more than two though). Some learn to avoid octopuses (and some don't) but they will usually keep aptasia under control (they won't eat the larger ones but do consume the tiny ones). These shrimp will pack and attack so I don't recommend more than two but with two you will sometimes get a little free tank food when they spawn.
Snails are another sometimes food item that are helpful to keep a tank clean as are a few hermits. I have found, however, that the blue legged hermits will kill some kinds of snails (but not all kinds) even if they have adequate shells. I have not seen this with the red legged variety and the red legged animals add a nice bit of color.
You can also try a few sponges (it is generally acknowledged that the red ball sponge can cause a major pollution issue if it dies but I have had no problems with the yellow ball variety and it seems to do better than most in my tanks).