An overflow box moves water from your display to your sump. It also sets your water level and improves surface skimming. The right choice depends on drilling, noise, and safety needs.

Internal overflow boxes (reef-ready drilled tanks)

An internal overflow uses a weir inside the tank. Water spills over teeth and enters bulkheads. This option needs a drilled tank or drilled back panel. It is the cleanest look for most reefs.

Plan drain size by real flow needs. A common target is 3–5x display volume per hour. A 75-gallon tank often runs 225–375 GPH through the sump. One 1-inch drain can handle this with margin, but tune matters.

Use safer standpipe styles when possible. A Herbie uses two drains and runs very quiet. A BeanAnimal uses three drains and adds redundancy. Keep the emergency drain dry during normal operation.

Common mistakes include undersized plumbing and poor weir length. Short weirs skim less surface film. Aim for a weir length of 6–12 inches on medium tanks. Keep the overflow teeth free of algae and snails.

• Set return flow so the overflow runs quietly, not at maximum.
• Use a gate valve on the main siphon, not a ball valve.
• Add a strainer on each bulkhead to block snails and fish.

For sump planning and drain safety, review our sump design basics. For quiet drain tuning, see Herbie vs BeanAnimal.

Hang-on-back overflow boxes (non-drilled tanks)

A hang-on-back overflow lets you add a sump without drilling. It hangs on the rim and uses a siphon to move water. This can work well, but it needs more checks. Siphon failure can stop drainage and risk flooding.

Choose a design with a strong, stable siphon. U-tube styles are simple and reliable. They restart well after brief power loss if sized right. CPR-style boxes rely on an airlift pump to remove bubbles. That pump becomes a critical part.

Match overflow rating to your return pump after head loss. A pump rated 800 GPH may deliver 450–550 GPH at 4–5 feet. Keep the overflow at 1.5x the real flow for safety. This reduces gurgle and bubble buildup.

Test power-off behavior before you trust it. Mark the sump’s maximum fill line. Shut off the return and watch the sump rise. Leave 10–20% extra sump volume as a buffer. Use a return nozzle near the surface to limit back-siphon.

• Clean the siphon path monthly to prevent bubble traps.
• Use an ATO to keep display level stable and reduce slurping.
• Install a high-water leak sensor near the stand base.

If you need help sizing pumps and head loss, read our return pump sizing guide. It helps prevent overflow mismatch and noise.

External coast-to-coast and modular overflow options

Coast-to-coast overflows use a long weir across the back wall. They skim surface film very well. They also allow lower water velocity over the weir. This often reduces noise and microbubbles.

Many hobbyists pair a coast-to-coast weir with an external box. Bulkheads pass through the back panel into the outside box. This keeps the display less cluttered. It also increases usable aquascape space inside the tank.

Modular weirs and low-profile internal boxes are another option. They fit tight spaces behind rockwork. They can be good for peninsula tanks and shallow reefs. Confirm the weir height matches your desired display water line.

When drilling, keep holes away from edges. A common rule is one hole diameter from any edge. Use proper bulkhead gasket placement on the wet side. Hand-tighten plus a quarter turn is usually enough.

• Target 1,000–1,500 GPH per 1.5-inch full siphon, with tuning.
• Keep emergency drains unrestricted and higher than the siphon inlet.
• Plan unions on all lines for easy pump and valve service.

Sources: Reefkeeping Magazine (drain system discussions), Bulk Reef Supply educational guides (overflow and plumbing fundamentals), manufacturer manuals for bulkheads and overflow boxes.

An overflow box is a safety system first. Choose the most reliable option your tank allows. Then tune flow, add redundancy, and test power-off behavior. Your sump will run cleaner and quieter.

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