For a non-permanent fix, you could use a bulkhead fitting to plug the hole.
For a permanent fix, you would want to use Weld-on acrylic cement to attach a larger sheet of acrylic to cover the hole. I would get a piece of acrylic at least 2" larger than the hole in every direction, but bigger would be even better. So that the bottom of your tank is flat, I would attach it on the inside of the aquarium and not underneath.
Check this link out for more tips:
http://www.tapplastics.com/info/adhesives.php?
If you have a tap plastics in your area, I would go in and talk to them about the best way to go about sealing the hole (if you want a permanent fix). They sell scrap pieces for really cheap and you can practice on those before working on your tank because once the weld-on sets, there is no going back...it's there for good.
Edit: Jay beat me to the punch.