Here's my advice on drilling glass
1. The glass can not be tempered. Tempered glass can not be drilled.
2. Ideally use a drill guide. Starting a drill bit on glass without one is difficult for the beginner. In addition, the guide gives you a reservoir for the water to keep the bit wet. If you don't have one, it is easily made from plywood or acrylic using a hole saw of the approximate size of the glass bit you are using. Here's a picture of an acrylic guide I use.
3. Using plumber's putty seal the guide to the tank and use tape (duct tape works well) to hold the guide in place.
4. Before you start to drill, put something below to catch the "hole." If you don't do this, the hole falls and cracks the opposite side of the tank.
5. Fill the reservoir with water and start drilling...with no pressure on the drill.
6. Stop and add water as needed. Be especially careful when you are close to breaking through to minimize glass splintering.....that is, no pressure.
Hope this helps.