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I have an old document I wrote on raising O. mercatoris that should help answer most of your initial questions. It is in word format and I can't attach it through TONMO email but if you will PM me your personal email address, I will be glad to send it. Clicking on my name will provide a link to send a PM.
For starters: Tank Size: 15 -25 gallons Food: fiddlers work well, Mine never ate hermits after acclimating well to the tank, ghost shrimp are too fast for the adults but are a good food if you feed it with a feeding stick. Equipment: fully cycled aquarium (3 months min) with live rock, filtration, an escape proof top and red ligthing
I have found that the mercs, especially the females will often den in a cluster of giant purple barnacles placed about 1/3 of the way up the tank. This set up will allow you to see them far more easily than if they take a live rock den. Females will often pick a den and stay there most of their lives. Males tend to move to different dens from time to time. Sourcing them is not an easy matter but we have seen more of them recently than in the last couple of years.
I've also heard of them eating snails... Is this true? I have easy access to a variety of hermits and snails, but no fiddlers.
And how much would they need to be fed daily? Could I just toss a few hermits/snails/fiddlers/etc in there, and let them hunt all day?
And I know of one guy who says he can collect them. Tom @ divertom.com. Has anyone here bought octos from him? I've talked with he and he is a good guy, just not sure if his identifications on the octos he sells are good or not.
Tom is often an octo supplier and one of the few that has actually kept an octo (albeit a LOOOOONg time ago). IDing is not easy for anyone when the animals are small. Most of his "collecting" is from the crabbers so you never know what will show up but Tom does a good job if they have a little size to them. Mercs are easier to ID than some of the others because of their coloration and large mantle to arm ratio.
They WILL readily eat hermits if you take them out of the shell (not an easy task - I have been told but never succeeded with limited trying that you can leave them out of the water and they will come out on their own but you would have to watch them closely so they don't dry up) and CAN eat them from the shells but mine never have, likely because they get an easier, hand fed meal.
As adults, they will sometimes take frozen, shell on pieces of regular shrimp. If you can get it to accept this eventually it will take the shrimp piece without the shell.
Snails are most likely going to go uneaten by you can put some in the tank and see if the shells become empty. Most of my octos just play with the snails but are rarely successful in getting a meal. If you can find some without and operculum (the little trap door), these may be a better opportunity to include in your feeding program. However, if you also put blue leg hermits in the tank, the hermits may kill the snails for their shells. The red legs don't seem to be as aggressive and I successfully keep them together in other tanks.
Octopuses are not main stream so ordering a specific species is almost impossible (there a couple of distinct animals that can be ordered but they fall in our, not for home aquariums category). In the past, watching eBay for a Caribbean dwarf has been the biggest success. These are usually a byproduct of collecting live rock from the LR farms. On occasion Philipp and Kara get them (KPAquatics.com) and they had two last week that came in through the shrimpers. Unfortunately, K&P usually have a waiting list and the animals they do get are claimed before they can be posted (a certain webmaster for their site is a bit piggy with this I will have to admit. She is hoping for two non-mercs next week ). I am hoping they will get more mercs this way as many can't keep the larger O. briareus they often find. If you do source one, it will likely be an adult. This is not so bad if it is a female as the hatchlings are one of the easier (but NOT easy) to attempt to raise.
Keeping with the plausible and not the possible, I suggest something 60 gallons or larger WITH as large a sump as you can fit. The water volume, especially over time, is important. The swimming space is minimal but has been accepted by the animals we keep. There are some of equal size that would need more mobility space. You can't create too much space but with a dwarfs, you won't see the animals in a more "natural" environment. This is true for the larger animals as well but we can't practically provide the kind of space where they can hide and never be seen.