DIY project

How to build a DIY aquarium overflow box

Intermediate8 steps$50

What you'll need

Acrylic sheets (cut to size)
PVC pipe (1-1.5 inch)
90-degree elbows
Bulkhead fittings
Aquarium silicone
U-tube or hose clamps
Drill + hole saw

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+).

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Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.