Best Overflow Boxes & Drain Kits

An overflow box is the connection between your display tank and your sump — the critical link that allows water to drain by gravity to filtration below and return via the return pump. Getting the overflow right determines whether your system runs silently and reliably or becomes a constant source of noise and flooding risk.

Internal vs External Overflow Boxes

Internal overflows are built into the tank itself — a section of the back wall is partitioned off with a weir and drilled bulkheads that connect to drain plumbing. This is the standard approach for reef-ready tanks and custom builds. Internal overflows are inherently reliable because they use gravity through drilled holes — no siphon to break, no U-tubes to maintain, no risk of air lock failure during power outages.

External hang-on-back overflow boxes are designed for tanks that cannot be drilled. They clamp over the rim and use a siphon tube to transfer water from the display to the external box, which drains to the sump. The Eshopps PF series and CPR CS overflow boxes are popular options. The trade-off is siphon risk — if the siphon breaks during a power outage or from air bubble accumulation, the overflow stops draining while the return pump may continue filling the display.

For undrilled tanks where drilling is not possible (tempered glass panels, rental situations, or tanks without accessible bottom panels), external overflows are the only option. They work reliably when properly set up and maintained, but they require more attention than internal overflows — checking the siphon tube for air accumulation, ensuring the U-tube stays primed, and testing the restart mechanism periodically.

Drain Configurations: Durso, Herbie, and Bean Animal

The drain plumbing inside the overflow determines noise level and redundancy. Three standard configurations dominate the hobby, each with different trade-offs between simplicity, noise, and safety.

Durso standpipe is a single-drain design with a modified pipe that introduces air at a controlled rate to reduce gurgling. It is the simplest configuration — one drain, one standpipe, minimal plumbing. The trade-off is noise: Durso installations produce a low hum or periodic gurgling that varies with water level and flow rate. Acceptable for fish rooms and garages, but often too loud for living spaces.

Herbie method uses two drains — a primary full-siphon drain tuned with a gate valve to run completely silent, and a secondary emergency drain that stays dry during normal operation but catches overflow if the primary clogs or slows. When properly tuned, the Herbie method is near-silent. The gate valve requires occasional adjustment as biological growth changes drain characteristics, and the primary drain needs periodic cleaning to prevent flow restriction from coralline algae and calcium buildup.

Bean Animal is the gold standard — three drains providing primary silent siphon, secondary open-channel backup, and tertiary emergency overflow. The triple redundancy means the system handles any single drain failure without flooding. Bean Animal setups require larger overflow boxes to accommodate three bulkheads and more complex plumbing beneath the tank, but the combination of silence and safety is unmatched.

Sizing and Bulkhead Selection

The overflow must drain at least as much water as the return pump delivers — otherwise the display water level rises until it overflows the rim. Size the drain capacity for 1.5 times the return pump flow rate to account for reduced efficiency as biological growth accumulates inside plumbing over time.

Standard bulkhead flow rates in gravity drain configurations: a 0.75-inch bulkhead handles approximately 150 GPH, a 1-inch bulkhead handles about 300 GPH, and a 1.5-inch bulkhead handles roughly 750 GPH. These figures assume full-siphon operation — open-channel drains move significantly less water than full-siphon configurations at the same pipe diameter.

Overflow Features Worth Having

Adjustable weir teeth let you fine-tune the display water surface level. Wider teeth reduce the risk of small snails and debris clogging the overflow and causing water to back up. A removable weir comb makes cleaning fast — biological film and coralline algae accumulate on weir teeth and gradually reduce flow capacity if neglected during routine maintenance.

Surface skimming efficiency is the primary reason overflows exist beyond simple water transfer. By drawing water from the surface, the overflow pulls the protein film, dust, floating debris, and oily residue that accumulates on the water surface. A well-designed overflow with correct weir height maintains constant surface skimming that keeps the display looking polished and allows optimal gas exchange at the water surface.

Installation and Plumbing Tips

For drilled tanks, ensure bulkhead fittings are properly gasketed and tightened — hand-tight plus a quarter turn with channel locks is standard. Over-tightening can crack the rubber gasket or stress the glass around the hole. Use Teflon tape on threaded fittings below the bulkhead but never on the bulkhead-to-glass connection, which seals via the rubber gasket compression alone.

Route drain plumbing with gentle slopes and minimal turns. Hard 90-degree elbows create turbulence, back pressure, and noise — 45-degree fittings or long-radius sweeps reduce all three while increasing effective drain capacity. Keep vertical drain runs as short as practical. The transition from vertical drain to horizontal run in the stand should use a gradual sweep, not a sharp elbow, to maintain laminar flow and reduce water hammer noise.

For hang-on overflow boxes, prime the siphon completely before starting the return pump. Trapped air migrates upward and eventually breaks the siphon. Test the siphon restart mechanism monthly — run a simulated power outage by unplugging the return pump, waiting for the siphon to drain, then restarting to verify the siphon re-establishes automatically. Discovering a failed restart mechanism during an actual power outage means coming home to water on the floor.

Common Overflow Problems and Solutions

The most common overflow complaint is noise. A drain that gurgles, slurps, or produces a constant rushing sound makes the tank unusable in living spaces. The fix depends on the drain configuration. Durso standpipes can be quieted by adjusting the air inlet hole size — drilling a slightly larger hole or adding a small valve allows fine-tuning the air-to-water ratio. For Herbie setups, the gate valve on the primary drain needs micro-adjustment until the drain runs as a full, quiet siphon with no air entrainment.

Micro-bubbles entering the display from the overflow drain are caused by air being pulled into the drain plumbing, typically at the standpipe or where plumbing fittings are not fully sealed. Check all plumbing joints below the waterline for hairline gaps. Apply silicone sealant or use solvent-welded PVC connections rather than friction-fit dry assemblies. A bubble trap baffle system in the sump catches any remaining micro-bubbles before they reach the return pump.

Salt creep around the overflow box — dried salt residue forming on the rim and nearby surfaces — indicates splashing or spray inside the overflow chamber. Adding a splash guard or adjusting the weir height to reduce turbulence eliminates most salt creep. While cosmetically annoying, salt creep also accelerates corrosion on any metal components near the overflow, making it worth addressing promptly.

Flow reduction over time happens as coralline algae, calcium deposits, and biological film gradually narrow the internal diameter of drain pipes and coat bulkhead openings. A semi-annual cleaning — disconnecting the drain plumbing and soaking in a vinegar or citric acid solution — restores full flow capacity. Ignoring this maintenance eventually causes the overflow to back up, raising the display water level above the intended operating point and increasing flood risk during power events or feeding cycles.

When selecting between overflow box brands, prioritize reviews and forum reports from hobbyists running the same box on similar-sized tanks over manufacturer marketing. Real-world performance data reveals issues like weir teeth that are too narrow for certain snail species, lid designs that vibrate at specific flow rates, and siphon mechanisms that fail reliably after six months of salt exposure. The reef keeping community has extensive collective experience with every major overflow box on the market — leveraging that experience before purchasing saves both money and frustration.