Overflow boxes can be internal and external.
Most of the time, the internal version is the better choice. You can customize it more easily, and it’s even more affordable than the external version. You also put this under the tank and you can cover it with a cabinet. But for these to work, the main tank needs a drilled hole.
So if your aquarium doesn’t have predrilled holes then the external version may be a better choice. It’s simply too bothersome to have to drill the hole in your aquarium yourself.
The external version may also be better if you don’t have enough space under your reef tank. This is usually a HOB (hang on back) unit. You can hang it at the back, and it’s not that obvious because you hide it with your corals and other tank décor.
This doesn’t need a drill because they use a siphon to suck the water over the side of the tank. Its main disadvantage is that in some rare cases, air can enter the siphon so the water stops draining to the tank. So when your return pump continues to put back water in your tank, eventually it will overflow.
That’s why you should check if the overflow can draw out any air that enters the siphon so it remains primed.
Finally, an overflow box is usually clear, so it’s easy to check how much water is inside behind the corals and the décor.
