Last Updated: May 21, 2026
What Is an Aquarium Overflow Box and Why Do You Need One?
An aquarium overflow box is a device that allows water to flow from a display tank down into a sump filtration system without drilling holes in the tank glass. It uses a U-tube siphon or an external chamber system to continuously transfer water to the sump below, where it passes through mechanical, chemical, and biological filtration before being returned to the tank by a return pump. Overflow boxes make sump filtration possible on non-drilled tanks and are widely used in both freshwater and saltwater aquarium setups. Here are the best options available.
1. CPR AquaFuge Hang-On Overflow Box
CPR overflow boxes are among the most trusted hang-on overflow devices in the hobby. They use a reliable double-sided U-tube siphon system and are available in single and double tube configurations for different flow rate requirements. CPR overflows are rated for flow rates from 300 to 1600 GPH depending on the model and work reliably when properly set up. They include a cleaning brush, airline tubing, and clear acrylic construction that makes it easy to monitor the water level and siphon condition at a glance.
2. Eshopps Eclipse Overflow Box
Eshopps Eclipse overflow boxes feature a modern, integrated design that is quieter and more visually clean than traditional U-tube overflows. The Eclipse uses a patented internal chamber design that maintains the siphon automatically and is resistant to siphon failure due to micro-bubbles — the most common failure point of hang-on overflows. Available in medium and large sizes for flow rates up to 2000 GPH, the Eclipse is a popular choice for serious aquarists who want reliable sump operation on non-drilled tanks.
3. Lifegard Aquatics Quiet One Overflow Box
Lifegard Quiet One overflow boxes prioritize noise reduction and ease of maintenance. Their quiet overflow design minimizes the gurgling and splashing noise that is a common complaint with hang-on overflows, making them ideal for display tanks in living areas where noise is a concern. They are available for tanks up to 75 gallons and feature tool-free cleaning access and a self-starting siphon mechanism. The Quiet One is a well-regarded option for hobbyists who prioritize low-noise operation in their filtration setup.
Buying Guide: Selecting an Aquarium Overflow Box
- Flow rate matching: Match overflow box capacity to your return pump's flow rate. The overflow must handle the full return pump output or water will back up in the display tank.
- Siphon reliability: The biggest risk with hang-on overflows is siphon failure, which can flood the sump. Choose models with anti-siphon failure features and always have an emergency shut-off on your return pump.
- Tank wall thickness: Verify the overflow box is compatible with your tank's glass or acrylic thickness. Thicker tank walls require longer U-tube arms.
- Noise reduction: Gurgling from overflow boxes can be reduced by adjusting the water level in the box, using a Durso standpipe in the sump drain, or choosing a model with a built-in anti-gurgle design.
- Drilling vs. hang-on: If you are purchasing a new tank, consider having it drilled for an internal overflow — drilled overflows are more reliable and cleaner than hang-on alternatives.
Final Thoughts
An aquarium overflow box is an essential tool for running sump filtration on non-drilled tanks. Choose a model with a flow rate that matches your return pump, prioritize anti-siphon-failure design features, and monitor the siphon regularly to ensure continuous operation. With a reliable overflow in place, the filtration benefits of a full sump system become accessible to any aquarium setup.
