In most cases, the ideal overflow is built into the tank utilizing a hole "drilled" into the tank inside the overflow. Aquariums with this feature are sometimes called "pre-drilled" or "reef-ready" tanks. If you are planning a new tank and haven't purchased it yet, I recommend you seriously consider purchasing a pre-drilled tank. It is possible to have an existing non-drilled tank drilled to your specifications as well, which is a good alternative for those that already own a non-drilled tank, or want to have the holes drilled in "custom" locations.
This type of overflow is quite simple and has few opportunities for failure. The overflow is designed to allow water to spill "over" it a few inches below the tank's rim. Because water will only spill into the overflow if the water level is above the top of the overflow, the water level in the tank will remain constant. Also, no water can drain from the tank after the water level lowers to the top of the overflow. This type of overflow can fail only if the drain hole or drainage pipe become clogged, thereby preventing water from draining into the sump, although it's very easy to prevent this from happening, and is a subject I will go into detail about in Part II of the article. Perhaps the only downside to a drilled tank is that the overflow will take up some space within the tank, thereby reducing the amount of usable area for the tank's inhabitants.
