Step-by-step
Step 1: Plan dimensionsExternal overflow box hangs over tank rim. Inside the tank: a small chamber with intake teeth. Water flows over the teeth into the chamber, through a U-tube siphon, out the back of the tank into the external chamber, through a bulkhead, down to sump.
Step 2: Cut acrylic chambersInner box: 4x6x6". Outer box: 4x6x10" (extends outside tank). Acrylic is glued together with weld-on cement (NOT silicone for the box construction).
Step 3: Drill bulkhead holeHole saw cuts through the bottom of the outer box. Insert 1-1.5" bulkhead fitting. Tighten with washer + gasket on both sides.
Step 4: Set up U-tube siphonU-tube spans tank rim - intake leg in display tank, exit leg in external box. Siphon must always remain primed (hold water).
Step 5: Test siphon primingFill U-tube with water before initial setup. Once water flows, siphon self-maintains. NEVER let air in - siphon breaks + overflow stops + tank overflows.
Step 6: Add aquarium-safe siliconeSeal acrylic seams with aquarium silicone. Cure 7 days. Water-test for 48 hours before using with livestock.
Step 7: Plumb to sumpPVC from bulkhead drains into sump first chamber. Use a Herbie or Bean Animal style overflow design for silent operation.
Step 8: Install + testHang overflow on tank rim, prime U-tube, start return pump. Verify continuous overflow flow + no air gurgles. Watch for 24 hours minimum.
FAQ
Are HOB overflows safe?
Less safe than drilled overflows because the siphon can break (air bubble). If siphon breaks: water keeps flowing out through return pump but nothing drains - tank overflows. Use a drilled tank when possible.
How much flow can a HOB overflow handle?
Depends on bulkhead + plumbing size. 1" bulkhead = 600-800 GPH. 1.5" bulkhead = 1,200+ GPH. Match to return pump rated flow.
Should I drill my tank instead?
YES if possible. Drilled overflows are silent + safer + more reliable. Use HOB overflow only when you can't drill (rented + glass tanks 40g+).