I found this video on youtube while I was looking for ways to derim my 5.5 gallons. I tried it and it took me 12 mins to get the rims completely off. It was so easy. I would use a knife that you don't mind ruining though. Mine was covered in melted plastic when I was done.

DIY experts, I have a question! Do you think it is safe to remove the top rim only of a 30g tank that's 36" long built from 3/8 inch glass. The DIY aquarium calculator here http://www.theaquatools.com/building-your-aquarium says that 1/4 inch would be fine with a safety factor of "3.8" - but...