47 Gallon Tank Journal.

Oct 13, 2005
I am starting this thread so that others can follow the process of me building up my 47 gallon tank for either a bimac, or a few O. mercatoris. I know that most octopuses are cannibalistic, but the new member, Danthemarineman, has enlightened me that these species do very well when housed together. How many O. mercatoris would everyone recommend for a 47 gallon with very good filtration and skimming? Danthemarineman said that he keeps over 25 in a 30 gallon, and 18 in a 20 gallon. His is temporary, because he sells them, but I think I could easily house or so in the 47 gallon. Of course, this is if cannibalism does not occur, which will be the main study of this tank. I also hope that by having this many individuals, I can keep photo as well as journal records of sexual reproduction, egg laying, and hopefully, raising the babies. BUT, as usual, i'm getting ahead of myself. I know that some will attest to me keeping known cannibals in a tank together, but I have heard of some species of octopus "schooling". I am getting these octopuses from Dan. I am going to attach some pictures, but it is not complete yet. I am going to litter around various snail and hermit shells for them to build dens with, or simply reside in them. I have about 15-20 lbs. of live rock, and plan on adding more ASAP. I'm going to pile the live rock on one side,creating a "slope". A few very nice pieces of these rocks have holes where the entrance is small, but it leads toa basically hollow rock inside. Very good for octopus dens IMO. All that's left now is to seal the tank. I am considering the possibility of utilizing the current deflectors that some people put over air vents that makes the air blow into the room rather than straight upwards, and attaching them from the filter output to the filter itself to seal this part. The rest is simply covering the front top, and putting weights on it. Not sure if I should add toys or not, since there will be more than one, I think they will have plenty of enrichment just by socializing among themselves. Also, I know that most dwarfs are nocturnal, but I hope to "transfer" them to come out during the day to eat. Can octopuses "over eat"? I want to have a constant supply of live foodin the tank for them at first, then gradually switch it to only having food in the tank during the afternoon/evening, when I am home. What is optimal temperature for O. mercatoris? There are also some aptaisia i'm working on getting rid of...Pics! The first is a full tank shot, the second is a turbo snail which I hope will soon be a home to an O. mercatoris, and the third is a dreaded aptaisia...


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I've always encouraged octo owners to keep journals. However, you may end up with just one octopus - we'll see.

One thing you might do is provide more hiding spaces - shells, short lengths of pvc pipe, small rocks.

Good luck with this,

People can succeed with keeping multiple cuttlefish in a tank, so I don't see why not.

I imagine with enough hiding spots they wouldn't see each other too often, unless they sought each other out, or stalked the same meal...

Don't keep so many your filtration can't keep up.

I wish you success.
Thanks! This is sort of a research thing on how people with very, very large tanks can successfully keep more than one octopus. I won't be too heart broken if I end up with one, because i'm not actually "buying" them, and they might be nocturnal. How many do you think this system can hold? It's been cycled for a looooooog time. About 10 months.
I wouldn't go too extreme... If they're the same species as my Einy I'd say 4 in a tank that big sounds reasonable. He seems pretty content most of the time hanging out near the shell he calls home, usually waiting for his food to come near him. He does occasionally but much less often track his food down though. This could just be a personal characteristic of him individually though, and not typical of his species, or his adaptation to living in a captive environment. (Keep in mind I've only had him for 4 weeks so I haven't witnessed some of his activities and this is the only experience I have with any octopus period.)
If others of his species share this behavior then I'd say they most likely would stay out of each others ways most of the time, probably just peeking out at each other waiting for something to crawl nearby. I'm willing to bet they don't stray far from their home in nature either though considering how small they are and the dangers they face out in the open.

Then again, they could demonstrate survival of the fittest and you end up with one fat octo. There's only one way to find out I suppose. It would be interesting to observe their behavior as a group and see what happens if they coexist peacfully, besides breeding.
Ok, I was getting 4 anyways, so I guess that works. Feeding will be expensive, but no worries, my reef sis tocked full with hermits and scavenging snails, so if worse comes to worse, i'll have a food source in my 75 gallon. How big is Einy? Dan said that his are quarter to half dollar size.
I assume "quarter to half dollar size" means legs extended in every direction, tip to tip. I'd say he's maybe 3 inches long, from the tip of his mantle to the tip of his front arms. I can tell he's a tiny bit bigger now than when I got him a month ago.

I found out about a place that sells hermits by the 100's, and super cheap. You should look into purchasing from them. If they're the same as Einy, he really prefers hermits to fiddlers. www.keyscritters.com
I assume he eats maybe 2 a day if that? I don't know what I would keep them in...maybe a small critter keeper inside the main tank, but they may injure themselves trying to get them out...hmm...I could keep them in my 75 reef...I think that would be best for me, because I doubt my mom would be happy with a food tank under the main tank, and plus that's where I keep filter pads, carbon, extra heaters/filters/powerheads, etc. 50 would last a looooong time.
Most of the time he seems to only eat 1 or 2 hermits per night. If he eats a Fiddler, he doesn't seem to eat the next night. I think the hermits are just easier for him to conquer and perhaps that's why he goes after them.

Between four of them I bet 100 hermits would go faster than you think, especially if the hermits are tiny. You could just put them in your reef tank, let them serve two purposes. Cleanup crew, octo dinner. That's what I'm doing with mine and it's working out marvelously.
Brock, just be careful in adding all of the octopuses at once. While i'm new to "octo care", I do know that everytime you add a fish your tank does go through a little shock. Even if your tank has been up for 10 months, the bacteria may not be as used to keeping up with all of that bioload..... Water changes are a must!!!

Good luck btw, i'm hoping to get a mercatoris or two from Dan also :P.