Just be sure when you are starting the hole you want the bit at an angle to the glass surface. It you try to start drilling the hole with the diamond hole saw flush to the glass surface it will be very very difficult to control the bit and it will want to "ice-skate" around all over marring the surface before it starts biting into and cutting the hole. Start at about a 45 degree angle to make it easier to get the hole started. Then later restart the hole squarely against the tank surface using the half area of hole that is cut into the tank to help keep the bit from skating around.
Soon you will see how easy it is. Good luck.
