If you want to attempt this yourself, it's pretty easy : you'll need this http://www.harborfreight.com/diamond-point-rotary-bit-set-20-pc-69653.html and this http://www.harborfreight.com/80-piece-rotary-tool-kit-97626.html for less than $20 bucks well plus tax would probably be $20 bucks.
1. You'll need to lay the tank down with the side up that you want the hole.
2. Have your friend/partner hold the hose or several cups of water and drizzle it onto the cutting area - be careful electicity and water do not mix very well-
3. Mark the hole size to be cut, using a marker but a china marker would be best
4. Grab a seat and chuck in one of the straight bits on the right side (link). Put the dremel on med speed and lay the bit at about a 45 degree angle to the glass.
5. This is where your helper with the water comes into play, just enough water to keep it wet and on the bit. It'll be daunting at first,maybe practice on scrap first, but this is the "ghetto" way with out a glass hole saw. Once you have a hole started just follow the outline, take your time. Took me about 10 mins on my first hole.
or you could take it OIAB and see if he has the right size
gl