Well, sad news. You're tank isn't large enough for an octopus, even a dwarf one.
You can have a small 10-15 gallon saltwater tank - a reef tank or maybe an invertebrate tank and continue to enjoy the octopus here on Tonmo. Then later, when you no longer live in a dorm, you can have an octo tank.
this is just my 2 cents but maybe a mantis shrimp for your 10 gal. there preety easy to care for. have a very interesting preditor/prey display. and they do exzibit some very interesting behavior. not sure but i think the tank has to be acrylic though.
Some have claimed success with a bimac in a 30, but people around here strongly recommend something bigger. Bimacs can get pretty big and need space to play and hunt--some people on the forums here consider it animal abuse to keep such an intelligent animal in that small of a 'kennel.' 55 gallons seem to be the practical minimum.
The consensus seems to be that octo keeping is quite a financial and personal commitment, and if you don't have the resources to do it right, you shouldn't do it at all. If you're limited to 30 gallons, there are certainly some options for you.
Smaller octo species have been kept in 30 gallon tanks, however it might be a little bit more of a challenge. Bimacs are preferred because they're captive bred (you know EXACTLY what you're getting and how much natural lifespan is left, you're not finding out from some teenager at the pet shop) and they're diurnal. If you want to get a dwarf species for a 30, you're going to have to get one wild-caught that might only have a few months left ahead of it (or it could be a baby of something that might grow BIGGER than a bimac!). And there are plenty of wildcaught octos available on the internet.
If you're set on a 30, you don't have to see this as discouragement, but a challenge. People seem to be using red lights to see nocturnal octos, and to be honest, most bimacs do die within a few months, anyway.
see if you can get your hands on a pygmy octo (octopus joubini) they sell them here http://www.caribbeancreatures.com/invrt_ostps.html im pretty sure one could get by in a 15 gallon tank. im not saying this is ideal but if youre dying for an octo it should work.
if anyone has an objection please post and sorry if im restating exactly what dan said.
(out of curiosity why do have a 15 gallon limit in your dorms?)
by the way this is my 100th post... yaay maybe i should get one of those "lives" i've heard so much about
Thanks for your input, I'll look into it and see if its do-able, but I'd rather not put an octo in a tank thats not large enough for it. The limit in the dorm is actually 10 gal, but I think I'd be able to get by with a little larger tank. The limit is because of the weight, as far as I can tell. Its a high rise building, with something like 20 floors, so if there is a lot of people with larger tanks (like 30-50 gal) the weight from all the tanks (which can be quite heavy) would put too much stress on the supports. Either that, or they feel like being bitches.
They probably care more about water spills than anything. Dorms = alcohol x roughhousing. Disaster equals the integral of that between 0 and the number of gallons of water behind plates of glass in the room.
I think unless your building is very old or you're in some part of the world without modern building codes, the weight of water shouldn't be an issue. A 75 gallon tank including water, live rock, stand and accessories probably doesn't weigh more than 800 lbs, which is about equivalent to 5 people standing in line. Put 5 of your friends next to eachother in your dorm room, make them stand there all year, and I don't think your floor would collapse.