We got two big rocks with lots of hiding places, standered biocube filteration ( with skimmer, of course ), and emerald crab that is a little bigger than the octo and a slightly smaller fiddler for it to eat. I've got 20 or so fiddlers in a bucket, and we introduced it by letting it climb in. And we have a 29 gallon biocube.
It is likely hiding in the rock (they are nocturnal and hide with the lights on) but check the sump as it is often a place they find inviting and the grid gives them access.
I am confused about the emerald crab though. It is bigger than the octopus? Emeralds don't get much bigger than the first joint on a pinky finger. If the fiddler is a male, remove or disable its "fiddler" claw (the smaller claw and the claws of females are not usually a problem).
It is a little bigger than the octopus, and we checked the sump and the fiddler is pretty small but if it's ABSOLUTELY POSITIVELY NEEDED than I guess I could take it out and fiddle with the fiddler I REALLY HATE touching them though. It feels pretty weird.
The the sump access on the all in ones are a bit hard to octo-proof. You can try covering the overflow with a very large netting (large to prevent blockage) or placing a very course (I have used the type for air venting) sponge (again, it must be course to allow water flow and minimize cleaning, a typical aquarium sponge will not work).
Yes, it is necessary to remove either the whole pincher or disable it. Most crabs will drop a claw on their own if it is held immobile (so holding it by the claw over the tank will work). I try to break off part of the thin lower part (to provide additional food) but often the crab will shed the claw before I can accomplish this.
I would not put any predator larger than an octopus in an octopus tank. The prey vs predator dynamic changes with size. Emerald crabs are not meat eaters but the size makes me wonder if what you have is actually an emerald crab.
Day three: We found our octo dead this morning. We assume stress due to the fact that the water was perfect and there were no marks from the crabs. I'm really bummed about it. We're going to try again fresh. I'm not sure if I want to keep this thread up or not. But I think I'll keep it up.
Because octopuses are different from other marine creatures and saltwater aquariums barely resemble freshwater tanks, we try to encourage new keepers to use best practice methods of aging a tank and acclimating the animal. We do lose some animals even under the best conditions but starting out with known setups helps minimize losses.
Since you are starting again, would you entertain reconsidering your tank environment? For ANY marine setup, even though your water was tested with no ammonia and no nitrates (I will assume that is what you meant by "perfect"), that does not tell you the tank is cycled (often the pronouncement of fish stores who are unaware that no biological process was ever started). Water never exposed to decaying biological matter will show as "perfect" until you add biologics. If you used dead rock, you will not have had the bacteria needed to start a tank cycle. You will need to add something to begin the process. Live rock, dead shrimp and pure ammonia are three options (live hardy fish are also used but are undesirable on a number of levels). Live rock is my preferred method but look up cycling with the other methods to decide which will work best for you.
A newly cycled tank (one that has shown a spike in ammonia followed by a spike in nitrites with both eventually registering 0 - roughly a month later) will be the first step in preparing a tank for your octopus but still is not ready for its intended occupant. After the initial cycle, the tank needs to continue building bacteria so that when waste is produced, it will immediately be converted and not poison the occupants. This extended cycle takes about 2 more months of adding a few clean up crew critters and overfeeding them (there are other methods of building the bacteria but this is the simplest). You can add snails, crabs, hermit crabs and/or serpent stars and urchins to feed. These can be left in the tank and become food (in the case of the crabs and sometimes the snails) as well as permanent residence (ignored snails and hermits and permanent tank mates serpent and urchin).
Introducing your octopus to a cycled tank should take about 3 hours. The goal is to S L O W L Y match PH, temperature and salinity. The method I like best is to half fill a bucket with tank water (adding new saltwater to replace the water you removed from the tank) and then placing the BAGGED octopus into the water. Using another container of tank water, I use a turkey baster to add the tank water to the bag at roughly a turkey baster full every 10 minutes to start and 5 minutes near the end. Once the bag is full, use the baster to drain off half the water and continue the process. I highly recommend getting a set of test strips that will show you PH (testing salinity with the water you take off is also a good idea). Test your tank water and (using a separate strip) your octopus water and do not introduce the animal until they match.
That's some solid advice. I didn't know there was a whole process to introduction, we just let her climb in. The thing with the ph testers is a good idea, I think we have some laying around. We used live rock for biological process and it turns out that when we tested the water with a refractometer earlier tonight the water was EXTREMELY SALTY at 1.029, that was awful to hear, but the folks at the store were so nice they let us use some of the finest RODI water around to fix the problem. I hope that I can learn a lot more about this before we get the new octopus. I'm gonna need info and lots of it. Please help.
I drip acclimate almost everything that goes into my tank, usually takes around one to two hours, and as @DWhatley says, test pH and salinity at least to make sure. You run a very real risk of killing your animal with osmotic shock if you don't account for these factors.
And today I am over the moon with joy, I got my new octopus! He's a male mercatoris and when we put in a fiddler crab it captured and started to eat it like 20 minutes after he swam out of the bag. I am so proud of him and it is a good sign that he is in a hunting mood. I'll start a new journal about him if he lives over a week.