How Does an Aquarium Overflow Box Work?

Q: What's the difference between a drilled (internal) overflow and a hang-on-back overflow box?

A drilled overflow is built directly into the tank — the glass or acrylic has holes drilled through it, typically into an internal chamber separated by a wall (often called a weir), and water spills over that wall into the chamber before draining down to the sump through bulkheads. This is the more permanent, typically more reliable setup, common on tanks built specifically for sump filtration. A hang-on-back (HOB) overflow box is an external box that clips onto the back of an undrilled tank, and instead of a drilled hole, it uses a siphon — a U-shaped tube that pulls water up and over the tank's back wall into the box, where it then drains down to the sump. HOB overflows let an otherwise standard tank run a sump, but they depend on that siphon staying primed (covered below), which drilled overflows don't have to worry about.

By Hektor Jorgo Published September 14, 2027 Updated June 14, 2026 5 min read

An aquarium overflow box mounted on the back wall of a tank, draining water down to a sump below

Quick Facts

What It Does: Lets water flow by gravity from the main tank into a sump below, where a return pump sends it back up to the tank
Two Main Types: Drilled/internal overflow (built into the tank, often with a weir) vs. hang-on-back (HOB) siphon overflow
HOB Overflows Rely on a Siphon: A U-tube siphon pulls water over the tank's back wall — this siphon must stay primed to keep working
Gurgling Noise: Usually caused by air mixing with water inside the drain pipe as it falls toward the sump
Durso Standpipe: A common drain design with a small air-intake hole near the top that reduces gurgling by controlling how air enters the pipe
Siphon Break: If air enters an HOB overflow's siphon tube, the siphon can lose its prime and stop pulling water until restarted
Flow Matching: The return pump's output should be at or below the overflow's drain capacity, or water can back up in the display tank
Emergency Drain: A secondary, higher-placed drain that activates if the primary drain clogs, preventing the tank from overflowing

An overflow box is the piece of equipment that makes sump-based filtration possible — and also the part most likely to make an unexpected noise or stop working at 2 AM. Understanding how it actually moves water makes both the normal operation and the occasional hiccups much less mysterious.

Short Answer

An overflow box lets water flow by gravity from the main display tank down into a sump (a separate tank or container, usually below, holding filtration equipment such as a wet/dry filter) — a return pump in the sump then pushes that water back up to the display tank, creating a continuous loop. There are two main designs: drilled overflows, built into the tank itself with a weir wall that water spills over, and hang-on-back (HOB) overflow boxes, which use a siphon to pull water over the tank's back wall on tanks that aren't drilled. Most of the common issues — gurgling noise, a siphon that stops pulling water, or a slowly rising water level — trace back to how air interacts with the drain pipe or siphon, and how well the return pump's output is matched to the overflow's drain capacity.

Drilled Overflows vs. Hang-On-Back Overflow Boxes

Drilled (internal) overflows are built into the tank during manufacturing — holes are drilled through the glass or acrylic, usually into an internal chamber separated from the main display by a weir (a wall that water spills over once it reaches a certain height). Water flows over the weir into the chamber, then down through bulkhead fittings to the sump. This is a permanent setup with no moving parts to fail and no siphon to maintain.

Hang-on-back (HOB) overflow boxes are external units that clip onto the back of an undrilled tank, letting a standard tank run a sump-based system. Instead of a drilled hole, they rely on a siphon — a U-shaped tube that continuously pulls water up and over the tank's back wall into the box. This makes sump filtration possible without modifying the tank, but introduces a dependency: the siphon has to stay primed (a continuous air-free column of water) to keep working.

The Drain: Standpipes and the Gurgling Noise

Whichever overflow type is used, water exits the overflow chamber through a drain pipe down to the sump. As water falls through this pipe, if the pipe is only partially full, air and water mix turbulently — this is the source of the gurgling noise that's extremely common in sump-based systems.

The standard fix is a Durso standpipe (or similar design): a drain pipe with a small air-intake hole drilled near the top, above the waterline. This hole admits a steady stream of air alongside the falling water, which — somewhat counterintuitively — produces a quieter, smoother flow than a pipe with no intentional air intake at all, by giving the air a controlled path rather than letting it mix chaotically with the water.

Siphon Priming: The HOB Overflow's Dependency

For HOB overflow boxes, the siphon tube needs to remain a continuous column of water to keep pulling water over the tank wall. If air enters the siphon tube — from a water level drop, a small leak in the tubing or fittings, or simply after the system has been off — the siphon loses its prime and stops pulling water, even though the rest of the system (return pump, sump, drain) is unaffected.

Most HOB overflow boxes include a way to re-prime the siphon, often by manually filling the U-tube through a dedicated port until the siphon catches again. If this becomes a recurring problem, a slow air leak in the siphon tubing or fittings is the most likely underlying cause — worth checking before assuming it's a one-off. Canister filters can develop a similar trapped-air pattern after maintenance; our guide on Fluval FX6 trapped air covers the same underlying issue in a different piece of equipment.

Matching Return Pump Output to Drain Capacity

In a balanced sump system, the display tank's water level stays essentially constant — the return pump adds water at the same rate the overflow's drain removes it. If the return pump's output exceeds what the overflow's drain can handle, water accumulates in the display tank faster than it drains, and the tank's water level slowly rises.

This is why overflow and return pump capacity are sized together, not independently — an overflow rated for a certain flow needs a return pump that doesn't exceed it, in the same way that matching a filter's flow rate to a tank's actual needs matters more than simply choosing the highest-flow option available. Most overflow systems also include an emergency (secondary) drain — placed slightly higher than the primary drain — as a backup if the primary drain becomes clogged, giving the system a second path to drain before the tank itself overflows. Overflow and return pump sizing are only part of the picture, though — the sump on the receiving end needs to be sized for its own set of requirements, particularly how much water it needs to absorb if the return pump stops; our aquarium sump size calculator guide covers that side of the system in detail.

Quick Reference

Drilled overflows use a built-in weir and chamber; HOB overflow boxes use a siphon to pull water over the tank wall on undrilled tanks
Gurgling noise comes from air mixing with water in the drain pipe — a Durso standpipe's air-intake hole addresses this
HOB overflow siphons can lose their prime if air enters the tube — most boxes have a way to re-prime
Recurring siphon loss often points to a slow air leak in the siphon tubing or fittings
Return pump output should be matched to (not exceed) the overflow's drain capacity to keep the tank's water level stable
An emergency/secondary drain provides a backup path if the primary drain clogs

Frequently Asked Questions

What's the difference between a drilled (internal) overflow and a hang-on-back overflow box?

A drilled overflow is built directly into the tank — the glass or acrylic has holes drilled through it, typically into an internal chamber separated by a wall (often called a weir), and water spills over that wall into the chamber before draining down to the sump through bulkheads. This is the more permanent, typically more reliable setup, common on tanks built specifically for sump filtration. A hang-on-back (HOB) overflow box is an external box that clips onto the back of an undrilled tank, and instead of a drilled hole, it uses a siphon — a U-shaped tube that pulls water up and over the tank's back wall into the box, where it then drains down to the sump. HOB overflows let an otherwise standard tank run a sump, but they depend on that siphon staying primed (covered below), which drilled overflows don't have to worry about.

Why does my overflow make a loud gurgling noise?

Gurgling is almost always caused by air mixing with water inside the drain pipe as the water falls down toward the sump — when a pipe is only partially full, water and air tumble together, and that turbulence is what produces the gurgling sound. The fix most setups use is a Durso standpipe (or similar design) — a drain pipe with a small air-intake hole drilled near the top, above the water line. This hole lets a controlled, steady stream of air enter the pipe alongside the water, which keeps the water flowing as a smoother sheet against the pipe walls with a stable air column down the center, rather than the chaotic mixing that causes gurgling. If your overflow has gotten noticeably louder over time without any changes, check whether the standpipe's air-intake hole has become partially blocked (by debris or buildup) — restoring that airflow often quiets things back down.

My hang-on-back overflow box stopped pulling water — what happened?

This is almost always a siphon problem — the U-tube siphon that pulls water over the tank's back wall has lost its prime. A siphon needs to be a continuous, air-free column of water to keep working; if air gets into the siphon tube — from a drop in water level, a small leak in the tubing, or simply restarting after the pump was off — the siphon breaks and stops pulling water, even though everything else in the system is fine. Most HOB overflow boxes have a way to re-prime the siphon, often by filling the U-tube with water manually (sometimes through a dedicated priming port) until the siphon catches again. If this happens repeatedly without an obvious cause, check for small air leaks in the siphon tubing or fittings, since a slow leak will cause the siphon to break again after each re-prime.

How do I know if my return pump is too strong for my overflow?

The overflow's drain needs to be able to keep pace with whatever the return pump is pushing back into the tank — if the pump outpaces the drain, water backs up in the display tank rather than draining down to the sump. In a balanced system, the water level in the display tank stays essentially constant: the return pump adds water at the same rate the overflow drains it away. If the return pump's output exceeds the overflow's drain capacity, the tank's water level will slowly rise until it either reaches the overflow's emergency drain (if it has one) or, in a worst case, overflows the tank itself. This is part of why overflow and return pump sizing are considered together rather than independently — similar in spirit to matching a filter's flow rate to a tank's actual needs, oversizing one component relative to the other creates a mismatch rather than simply 'more is better.'

Sources & Further Reading

About the Author: Hektor Jorgo

Co-Founder & Marine Biologist

Hektor is a co-founder of Sea Life Planet and has kept reef and freshwater aquariums for over 15 years. He holds a background in marine biology and focuses on species care accuracy, water chemistry, and tank husbandry.