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Lemme say this before someone else rips into ya (), a 24 gallon is not appropriate for an octo for a number of reasons.
Please stick around, read some posts, learn some more about these amazing creatures, and then try again with a larger tank (I think the minimum acceptable size is 50 gallons for most species). I'd recommend heading over to the Octopus Care Forum: http://www.tonmo.com/forums/forumdisplay.php?f=8
Welcome to Tonmo. Once you've read all of the different papers, you'll probably have a better idea of just what you are in for when you start keeping cephs...they are a far and away different from any other saltwater animal, and extremely rewarding to observe.
I saw quite an odd comment from the www.deltecusa.us 's Tank of the month winner, David Saxby.
When asked what he would do if he had to start another tank completely from scratch he said chuck in an octopus.
"I'd add a Sea urchin or two which are a great "clean-up crew" to help with the biological balance, plus a little Octopus to catch any Mantis shrimps and crabs brought in with the live rock."
This from a man who has amazing reef tank, (3700G system) - struck me as a rather weird thing to say.
There are some dwarf species out there that might fit in a 24 gallon. TONMO'ers typically recommend about 30 gallons for a dwarf octo, however. These limits aren't just to give the octo some space, but octos have very high metabolism, veritable ammonia-factories (up three times as much as a similarly-sized fish!). Having a big water system is important to keep it from getting overloaded. Given that, if you've got some experience, you might be able to push it a little with a dwarf in a 24.
The big problem with dwarves is they're kind of hit-or-miss. They're very short lived (months, not years!) and you can only get them wild-caught. Further, the people that collect and sell them don't know much about them, so often times you'll get one (maybe you paid $40 for it and another $30 for shipping) and its already a grown adult with only a few weeks of life left in it. Add to this octos are usually pretty shy for their first month or so in their new home, and even after that they're usually nocturnal.
For these reasons the most recommended octopus is the California mudflat octopus, O. bimaculoides, often called a bimac. This animal is aquacultured, meaning you get it knowing exactly how old it is and it already is used to captivity. They will still be shy for the first month or so, but they will come out during the day after that. The problem is this gets to be a big critter. If you want one of these, you should get a 55 gallon tank at least. Most bimac keepers like to have a 75+.
Yup you do that and juuuuuuust keeeeeeeeeep waaaaaaiting!
I will however try to get some upto date pics of our boy, but he's been sulking in the back of his drain pipe bedroom for a week (we had the absolute gall to lower the water level so we could get in the tank to remove some gungy gravel!
In the UK it is a very common practice and many shops etc recommend to people that they put in live rock and then an octopus to eat any mantis shrimps then get rid of octo to then get the fish and inverts etc.
ceph keeping isn't being taken seriously in the UK yet