Aquarium Overflow Types

There are three main types of aquarium overflows: Built-In, OverflowBox, and Custom Drilled. Each of these have their pros and cons. Unfortunately, this is one of the many details in setting up an aquarium that can’t be changed later. So it is vital that you fully understand each of them so that you can decide which works best for you and your system.


Built-In overflows, also called reef-ready, are tanks that are drilled by the manufacturer and have built-in overflows.


  • No risk of cracking the tank while you drill
  • Very reliable


  • Higher cost for the tank
  • You can’t set the tank down on its bottom once you install the bulkheads
  • The overflows take up space in the tank
  • If anything gets into the overflows it is hard and very annoying to get it out (fish, net, etc.)
  • The overflows are either in the corners where you can see inside them, which is ugly, or
  • The overflows are centered on the back where you can’t see in them if you need to

Overflow Box

Overflow boxes are manufactured separately and hang on the back of the tank. There is a box on the inside and a box on the outside. There is a tube that connects the inside with the outside so that as the water level inside rises due to extra water being pumped in from the sump, it siphons to the box on the outside and then drains down to the sump.


  • Don’t require any drilling, so they can be used on a tank that is tempered, already running, or you are just too nervous to drill the tank
  • Take up less space inside the tank than built-in overflows


  • Not all overflow boxes are equal, some are better and more reliable than others
  • They aren’t cheap
  • If they lose siphon they don’t work at all and the tank will flood

Custom Drilled

Some stores will drill tanks for you (usually with no guarantee that they won’t crack them), but most people who get custom drilled tanks drill the tanks themselves. This is not as scary as it sounds. It is just a matter of getting a diamond hole drill bit, keeping the glass wet, starting at and angle before leveling out, and then just going slow and steady (keeping it wet the whole time).


  • You get the exact size holes you want exactly where you want them
  • Just as reliable as the built-in overflows
  • Don’t take up nearly as much space in the tank as built-in overflows
  • Can be much cheaper than paying for built-in overflows
  • Can look really good with internal overflow boxes (covers)


  • You have to drill the tanks (or pay a store to with no guarantee)