There is a third way. First some clarification on terminology:
- an "external" overflow as used in this thread is more commonly known as an HOB overflow. This is the type of overflow that Lifereef makes.
- an "internal" overflow usually consists of a drilled hole or two enveloped by an internal overflow box.
There is another kiind of external overflow that involves no drilling and no u-tubes. Here's mine:
Glass is notched out of the top of the back pane and the water overflows into the attached external glass box and down to the sump via the Herbie pipes. No chance of flood, no unsightly boxes inside the aquarium, and easily Herbie-able. Does cost a bit more to build, though. My mistake is not going wider or even coast-to-coast.