Overflow boxes route water from the display tank to the sump. Internal overflow: built into the back/corner of the tank (drilled holes through the bottom or back glass). Pros: quietest, cleanest look. Cons: requires drilled tank. External overflow box: hangs over the rim, U-tube siphon over the rim. Pros: works on undrilled tanks. Cons: siphon can break (catastrophic - sump dries, return pumps water onto floor), more visible. Herbie + Bean Animal plumbing: 2 + 3 drain configurations that eliminate gurgling. Bean Animal = silent, redundant siphons. Sizing: 2-4x return pump GPH for safety margin. Common issues: snails crawl into overflow + die (use a screen), bubbles from inadequate drain pipe diameter, gurgling from poorly-designed standpipes. Drilled tank vs HOB overflow: serious reefers always drilled. HOB overflow is acceptable for nano tanks only.
Reviewed by the Fast Aquatics husbandry team · Updated May 2026
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