Nancy's bimac care sheet at http://www.tonmo.com/cephcare/BimacCareSheet.php says 65-72F. I think the prevailing belief is that lower temperatures seem to be associated with longer life, so chillers are recommended... you might want to search for "bimac chiller" or similar.
This thread http://www.tonmo.com/community/index.php?threads/8077/ says DHyslop kept Mr. Octopus around 70, but without a chiller so up to 80 on hot days, and he was a very long-lived bimac... possibly the low temperatures in Zyan's octopus farm (see post #13 there) early in Mr. Octopus' life were more important than the adult tank temp.
How old/big is your octo, and where did it come from? Are you sure if it's a bimac? I believe the O hummelincki that are sometimes sold as "pygmy bimac" or "Caribbean bimac" are more tropical....
ya its a bimac, i couldnt figure it out what kind until it was in the tank and i saw the fake eye on each side with the blue ring, its mantle is the size of about half a ping pong ball. the LFS doesnt know what ocean it came from but just said its an octopus from the ocean lol. He looks pretty small, he has been hiding in the corner/ back of the tank since i put him in yesterday, he ate only 1 krill though he pushed away the rest of the krill with his arm. I named him cortez after the sea of cortez, its kinda cheezy but its funny.
Other octos besides bimacs show the blue eyespots, like O. hummelincki, as suggested by Monty. You might check them out to be sure. I'd say if yours has a habit of looking "spikey" fairly often, it's probably hummelincki (aka filosus). Pictures please!?!?
to see if your new friend looks similar. The importance of determining which of the two - Caribbean Two-Spot (hummelincki) vs Pacific Two-Spot (bimac) - is the tank temperature settings. The Pacific octo needs colder temperatures but the Caribbean animal would not likely fair well at 70 degrees as it lives in shallow, warm water reefs.
I would keep it at about 75-76 degrees until you're absolutely sure it's a bimac. There are a several different species with eyespots on the market right now. It's rare that bimacs are sold unless someone is breeding them, and we are unaware of that.
Compare the eyespot with the one shown in my Bimac Care Sheet - it has to look like that to be a bimac. It's also quite difficult to see the details of an eyespot when the octopus is quite small. So what do you think?
i think Nancy's list is right on, however there's a LOT of wives' tails regarding keeping octos. I had a California 2-spot that i found here in local waters, in my 73-degree water w/ the smallest chiller money can buy, and he did fine (and was old/large when i found him). i think that, like a lot of organisms, they adapt. and, if you consider that in the wild in a place like SoCal, they're in water with a HUGE range of temperatures - colder, deeper areas around 58-64 degrees, then in smaller, warmer pools of water like when i found mine. There was BARELY any water and he was under a rock...the water felt like 78 degrees at the coolest! i was amazed he wasn't dead...but like i said, they adapt to whatever pool of water they're in.
also, people say you need a huge amount of water which isn't entirely proven. At the Cabrillo Aquarium in San Pedro, CA, i visit a 7 year old 2-spot Bimac which is HUGE! Its mantle is about the size of a squash with very long arms, and they have her in a 60 gallon tank. so, nowhere does it say an octo needs a ton of water to survive. Just look at the very little amount of water that are in tidepools that they love to hang out in.
i think overall the saltwater hobby tends to push people into buying a lot of unnecessary stuff. there's a guy in KoreanTown (LA) who actually has LIVE, HEALTHY saltwater fish in systems with NO filtration systems...only aerators...unreal, i know, but....he swears by it and he's a wise, 80 year old Japanese fellow...interesting stuff though, either way!