Natalie,
You might want to add some cleanup crew right away for two reasons. One octopuses are messy and the waste is going to put an instant load on the tank and two, adding animals later is more likely to make them snacks if your octo is accustomed to anything you place into the tank being supper (so anything you DO add/replace, do so when the octo is asleep and definitely no where near feeding time). This applies mostly to hermits and snails and they are not safe from any octopus but are worth putting in and replacing (and will give a small food supply until it is used to being fed). Keepers have mixed results with octopuses eating them. None of mine seem to bother the red legs with the exception of O. vulgaris (I think they will eat almost anything that is in the Echinoderm phylum). Snails have mixed results as well, most typically, if the snail has an operculum (hard foot it uses as a door), they survive, especially if the octopus has other options (again I exclude O. vulgaris
).
My favorite octo-safe cleaner is a brittle star (I usually have 2 per octopus) and the one I like the best and have in all my tanks is commonly called a red brittle star (
Ophiocoma wendti). These are, hardy, long lived, fairly active, can be hand fed and often brightly colored. Sometimes they will come out during the day and they often den with my octopuses (if I can't find the octo, I will look for Pesky, the name I give all the reds). Another in the sea star family I like to keep that will consume left-overs (in spite of what some of the literature says) is the red thorny sea star (Echinaster echinophorus). These are day active and can be very brightly colored (somewhere I have a video where one insists on being the star of an octopus vid). This is the only starfish I have had absolute survival success with (save one unknown nocturnal I have had in the nano for over a year). Octopuses won't eat echinoderms but some are better than others. Avoid anything with sharp spines (rock urchins for one, pencils seem to be fine - cautuion they have their own peculiarities and may eat things you don't want eaten but won't hurt or be disturbed by the octopus), sand dollars don't seem to do well in aquariums and you have to be very careful with cucs. Nudis, on the other hand, will most likely be eaten before the day is out.
If you want a little color, gorgonians are fine but place them out of any obvious paths of travel. Octopuses do not go around things! Adding them later is OK but the octopus is likely to move it or even be annoyed with the addition to its territory. (
).
If you watch Maya's video in the above link you will also notice polyps. These are more difficult to recommend. The ones I have in the octo tanks are propagated from two sets of probably the same species that I have found octo safe but I don't have a name. Many (possibly most, including zoanthids) have too much sting to place in the tank. I have had to remove others and only use these now. If you do try some, add them on their own rock that can easily be removed. If you see any kind of touch and withdraw reaction, take them out.
With your permission (and a lot of luck), I will move this thread to the journals section when your new guy arrives so you will have a full log of this journey. If you would rather journal the animal separately (hoping you are going to keep us updated), start a new thread with its name (I can change that part later if you don't have one immediately, poor Onn kept his Octopus with No Name status because that is what we call him), use unnamed, to be named or something similar, followed by a dash and the species (or what you are expecting). Any moderator can change the subject title later on request.
Can't wait to see your first animal and read your reactions.