Types of Aquarium Overflows: Hang-On-Back, Internal Box and Corner
Every sump-fed aquarium needs an overflow: the part that skims water from the surface of the display and sends it down to the sump. Skimming the surface matters — that's where films and floating debris collect. The type you choose affects how much flow you can run, how much room it takes up in the tank, and how it looks.
Hang-on-back (HOB) overflows
A hang-on-back overflow hangs over the rim of the tank and moves water out over the edge. Its main appeal is that it doesn't need a drilled tank. The trade-off is that it relies on keeping a siphon going, so it needs to be set up and maintained carefully.
Internal overflow boxes
An internal overflow box sits inside the display, usually against the back wall. Water spills over the comb at the top, collects in the box, and flows down a drain pipe through a bulkhead in a drilled hole to the sump. This type needs a drilled tank, but there's no siphon to lose, which makes it the more reliable option. Ours are 3D printed in PETG and have an optional cover to keep fish and snails out of the drain.
Overflow box with the optional cover
Corner overflows
A corner overflow tucks into a back corner of the tank, so it takes up less visual space. Like an internal box, it drains through a bulkhead in a drilled hole, so it needs a drilled tank. Ours come in left and right versions to suit your plumbing.
How big should my overflow be?
Pick an overflow rated for the flow you plan to run through your sump. Here are the ratings for our boxes, all internal overflows for drilled tanks:
| Box | Approx. flow | Tank size (approx.) |
|---|---|---|
| Overflow Box 4"x4" (x3 / x4) | 660 / 780 GPH | 65 / 75 gal |
| Overflow Box 6"x4" (x3 / x4) | 780 / 900 GPH | 75 / 90 gal |
| Overflow Box 8"x4" (x3 / x4) | 960 / 1080 GPH | 90 / 100 gal |
| Corner Box 4"x4" (x3 / x4) | 430 / 520 GPH | 40 / 50 gal |
| Corner Box 6"x4" (x3 / x4) | 600 / 660 GPH | 60 / 65 gal |
| Corner Box 8"x4" (x3 / x4) | 720 / 780 GPH | 70 / 75 gal |
Next steps
Once you've picked an overflow, plan what happens at the other end — see our post on filter sock silencers and browse the For the Sump collection.