This is trying the difficult way with a Dremel and very small diamond bits (like your dentist uses)...
Cut the top off of a plastic tumbler-type cup with the Dremel.
Getting the supplies ready to go through the back of this 125g tank. (the back of the tank is painted navy blue)
The plastic ring is duct taped to the tank resulting in a very good (not perfect) watertight seal. This will do a fine job holding the water keeping the tank and bit cool.
Almost all the way through the glass. This is the slow way and results in a hole that is not perfectly circular. Did I mention that it is painfully slow and hurts your ears yet? I will not ever drill glass with my Dremel now that I have found the better Diamond Hole Saw bits for my cordless drill.
