When drilling aquariums, make sure the glass you are about to cut a hole into is not tempered. Most aquariums over 30-gallons have a tempered glass bottom, and cutting into these will result in a large crack forming nearly 100% of the time. The way aquarium manufacturers get holes in the bottoms of their aquariums is to drill it prior to the tempering process. Other aquarium makers use extra thick glass, which doesn’t require tempering but costs quite a bit more. Regardless of your aquarium, if you decided you need extra holes in your tank, contact the manufacturer and ask which panes are tempered and which are not. It’s better to be safe than sorry, especially since most aquariums are very expensive.