building your own tank

Bulding your own aquarium is a lot of fun, and is really not that hard. You just need to get the basics right. Tanks 2' and up need bracing. While constructing make sure to get enough silicone between connecting pieces "seams". And masking tape/"paper tape" make life a whole lot easier with regards to cleaning up afterwords.
I don't think you'll be able to aford the glass for a 12' tank, since that's more lenght than a standard glass sheet can supply. Bring it down to 10' (or what ever the length of a standard sheet of glass is). 1/2" (12mm) thickness will do just fine.
Please just remember to sit the tank on a piece of polystyrene (same size as the tank's footprint).

Here's an interesting link with regards to glass thickness. Safty factor of 3 is safe, any thing under 2.5 is playing with fire with the amount of water your planing to hold. But I have to admit that I have 2 25G tanks built with a safty factor of 1.9. They're stable enough for me... But if they should go for some reason, 25G is a lot less water to mop up than 300G.
http://saltaquarium.about.com/cs/aquariumdiy/l/blcustomtank.htm

Moving a tank that size can really be a problem. Think about going around corners, up stairs, etc. It's like the stories of the guy who builds a boat in his basement, then finds out there's no way to get it out.

You will find that buying plywood or glass over 8' will be amazingly expensive. You will need a gang of people to handle the glass.

You don't need to build a tank all the way to the wall just because there isn't glass on the side.

Building two 6' tanks, or an 8' and a 4' is probably a better solution.

I'm in the process of my own DIY. It's been, shall I say, a process There is a ton of info on the web.

Build up info
http://www.plantedtank.net/forums/diy/28182-start-my-130ish-gallon-corner-setup.html

or just straight to the pics
http://webpages.charter.net/tusk/tusk/pets/project_pics.htm

too bad I broke a bone in my hand a 2 weeks ago and I still have 6 weeks to go until I can get back to work

i personally havent tried. i heard pretty much the most difficult/expensive part is buying and placing the braces. if you only plan on making one tank the cost of the c-clamps make it more expensive than just buying a tank. if you end up doing it i would love to see some pics!!

Tools shouldn't be that much of a problem. A good plywood source will accurately cut the pieces for between $.50 and $1.00 a cut- certainly cheaper than any decent saw would cost. (If you go to a big box lumber store, let them know you will NOT accept any mis-cut pieces!) You don't need any clamps- all the joints are held together with screws until the glue sets. A power drill is a must- it doesn't have to be an expensive cordless one! Cutting a hole in the front will be the most trouble for a beginner, but you can use a drill and jigsaw, or a router. Paintbrushes and a $1 caulking gun would round out the tools.

I'd suggest getting square-drive screws made for wood, rather than drywall screws. Stainless-steel ones aren't THAT expensive if you buy them from McFeely or other mailorder source.

I've built my own 150 gallon tank using plywood and having glass only as the front pane. This tank has worked very well, but it's not that much cheaper than buying a completed glass tank once you figure in the time and effort especially. If you really want a tank viewable on 3 or more sides, then buy a tank from a vendor, but if you want a "custom" tank with a non-standard size, shape, and length, then build it yourself! You have to enjoy doing this kind of stuff yourself to make this job worth it. Regardless of what you've heard, the tools you need to buy to even begin this process can be a serious "out-of-pocket" expense". There are cordless drills, at least 1 circular saw (if not a table saw/etc), jig saw, power-sanders and various sandpaper, screws, various drill bits and counter-sink drill bits, special epoxy, disposable paint brush and other related equipment, Respirator (don't skimp on this one), large dry work space, friend/partner to help you move the parts into the house (you cannot use these epoxies in your house!), and make sure you can get a pane of at least 1/2" glass made at the length you want (no need for tempered...it just costs more and gives no added strength). I'm sure I'm forgetting something so be prepared for additional unforseen expenses! Otherwise it can be a very enjoyable and rewarding experience!