Drilling Your Reef Tank for an Overflow: A UK DIY Guide

Making a Reef-Ready Tank

Got a tank that needs a sump? The cleanest approach is drilled holes with bulkhead fittings — no hang-on overflow boxes, no siphon tubes that lose prime at 3am. It sounds intimidating, but with the right process it's straightforward.

Can the Tank Be Drilled?

Critical first question: is the glass tempered (toughened)?

Tempered glass CANNOT be drilled. It shatters instantly. No exceptions.

How to check:

• Ask the manufacturer with the model number
• Look for a stamp — tempered glass sometimes has a small etched mark in the corner
• Polarised light test — tempered glass shows stress patterns as rainbow bands through polarising filters
• When in doubt, assume the bottom panel is tempered. Side and back panels are usually non-tempered

The most common DIY approach is drilling the back panel near the top, avoiding the bottom entirely.

What's Needed

• Diamond hole saw bit (25mm and 32mm are common, £10–20 each)
• Variable-speed drill — slow, steady RPM is essential
• Bulkhead fittings with proper gaskets (£5–15 each)
• Masking tape, spray bottle of water, plumber's putty for a water dam, safety glasses

The Process

1. Mark the hole position. Keep the centre at least 50mm (ideally 75mm) from any edge — glass is weakest near edges
2. Create a water dam with plumber's putty around the mark. Fill with water for lubrication and cooling — drilling glass dry cracks it
3. Apply masking tape on the inside to reduce chipping on exit
4. Drill at low RPM with light, consistent pressure. Let the bit do the work
5. Keep it cool — maintain water throughout
6. Slow down at the end — the exit point is where chips are most likely
7. Clean up rough edges with 400-grit wet/dry sandpaper

Installing Bulkheads

1. Gasket on the outside of the tank
2. Insert bulkhead from the inside
3. Thread nut onto outside over gasket
4. Hand-tighten, then a quarter turn with pliers. Do not overtighten — cracked glass or deformed gaskets result
5. Fill slowly and check for leaks

Overflow Design Options

• Internal overflow box — siliconed to the back wall, most common and reliable
• Coast-to-coast overflow — full-width weir, quieter, more complex
• Bean Animal overflow — three drain pipes for virtually silent operation, the gold standard for noise

Common Mistakes

• Drilling tempered glass (always check first)
• Holes too close to edges (minimum 50mm clearance)
• Pressing too hard (let the bit cut at its own pace)
• Dry drilling (always keep the bit wet)
• Overtightening bulkheads
• Not leak-testing with freshwater for 24 hours before adding livestock

DIY or Pay Someone?

For anyone reasonably handy, it's about 30 minutes per hole. For those who'd rather not risk it, many local fish shops and custom builders offer drilling at £20–40 per hole.

Browse reef-ready tanks and plumbing equipment from UK sellers — pre-drilled tanks on the used market are a smart alternative.

Anyone tackled this DIY? Tips and lessons learned are always welcome.