Is your tank glass or acrylic? In either case, the bulkhead will drive the size of the hole you need to drill. If your tank is acrylic, you can purchase a hole cutter (that attaches to any drill) from a hardware store. We just bought one from Lowe's for my 1.5" bulkhead... it requires a 2 3/8" diameter hole. For glass tanks, that requires a special diamond-coated drill and enough water and suction cups to prevent cracking. I had a friend down south drill my 29 gallon glass tank.