Drilled the 2 holes in my aquarium today, the first one came out fine, the second I guess the bit didn't stick into the groove very well and it "cut" a little bit more out of the side. It isn't chip out, and its on the outside of the glass and is very smooth but the glass is thinner there from it being partially sanded out. The gasket still covers this area so I don't think it really matters and it's at the top so it won't be taking any real water pressure. I'd still like to get some opinions however. This is a 20g long.