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How to Drill a Glass Aquarium for Overflows and Returns

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Last updated on November 17th, 2023


Wall to Wall Reef Tanks

Why Should You Drill Your Glass Aquarium?

It's common for to decide to add an overflow to a standard aquarium rather than buying a Reef Ready tank. Often times it comes down to two reasons. The most common is that you got a REALLY GOOD DEAL on an aquarium, but it wasn't drilled yet. Another is that the location of the hole in a standard Reef Ready tank didn't fit your system.

Regardless of the reason, I'm going to talk to you about how to drill a glass aquarium. I'll show you what I did for my frag setup, and what I would do differently. There are also some things you ABSOLUTELY MUST NOT DO! I will talk to you about some of that here as well.

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Benefits of A Drilled Aquarium

The number one benefit of having a drilled glass aquarium is that your plumbing is more secure. Obviously there are a number of benefits of having a sump. Without a drilled aquarium, you are stuck with hang-on-back overflow options. Those these work, there is an added risk of flooding if the HOB overflows' siphon breaks (bubbles in the tube). This is where having a drilled glass aquarium is an advantage, because there is no siphon required.

First Things to Consider

Before showing you how to drill a glass aquarium, I want to ensure you consider a few details. The first thing is that you should NEVER attempt to drill tempered glass. It is practically guaranteed to shatter. Adding to this point, the bottom glass of most aquariums is typically tempered; so DO NOT try to drill the bottom glass. You will need to do some additional research about if the other glass is or is not tempered on your tank.

The next detail is that you have to be ready to accept the possibility of this tank shattering. When you drill a glass aquarium, it is possible for the entire pane of glass to crack. If this happens, you will need a new tank. Plan ahead and don't put all of your eggs in this one basket.

Next, you CANNOT drill an aquarium with water in it. The pressure from the water will cause the glass to crack. I repeat, DO NOT drill an aquarium with water in it!

Finally, remember that these holes are permanent. You will want to make sure you plan ahead, spend the extra money for the right equipment such as overflow boxes and returns, and not just what is cheapest. You can't undo a hole cut in a glass aquarium, so plan ahead, measure twice, cut once.

Planning to Drill a Glass Aquarium

Before you actually drill the glass, you will want to plan where the holes need to be. The best way to do this is to purchase the Overflow Box, Bulkheads, and Sump Return Fittings. This way, you have the actual equipment that you can plan with and trace, rather than trying to measure and mark everything yourself.

Now that you have these, and the tank, layout where the overflow and return line's bulkhead need to be on the tank and trace the inside hole with a permanent marker. Make sure to give yourself at least 1/4" from the trim of the tank, do not try to get anything right up against trim or edges of the tank. If there is any movement with the drill (and there will be at least a little), then you may not be able to fit your equipment.

Now that everything is planned out, you will need to set up a place to actually drill the tank. The ideal location is outside because you need access to water such as a hose. Additionally, remember there will be small amounts of glass from drilling the glass aquarium itself. So make sure to drill the tank away from where others may step on any of this glass.

What do you Need to Drill a Glass Aquarium

Now that you have the Overflow Box, Bulkheads, and Sump Return Fittings, and a place to drill the glass aquarium, you will need just a little bit of equipment. First, the obvious, a simple Cordless Drill. Nothing special. If there are multiple setting (such as the "1" or "2" options on Ryobi Drills), use the slower option to avoid causing too much heat.

Next, you need a diamond coated hole saw bit. A regular drill bit cannot cut glass, so you will need the right size diamond coated hole saw for the bulkheads you are cutting a hole for. See the table below to find the bulkhead and bits needed. In addition to this, you will need a standard Garden Hose and something like a brick to weigh it down.

Bulkhead Diamond Coated Drill Bit Size

Bulkhead SizeDrill Bit Size
1/2" Bulkhead1 1/4" (30mm) Diamond Hole Saw
3/4" Bulkhead1 3/8" (35mm) Diamond Hole Saw
1" Bulkhead1 3/4" (45mm) Diamond Hole Saw
1 1/4" Bulkhead2 1/8" (53mm) Diamond Hole Saw
1 1/2" Bulkhead2 3/8" (60 mm) Diamond Hole Saw
2" Bulkhead3" (75mm) Diamond Hole Saw

How to Drill a Glass Aquarium

As for how to drill a glass aquarium, the process is quite simple. Just remember to GO SLOW. Set up the hose so that there is a moderate flow, but not blasting everything in water. Think about an 8 inch stream without a ton of air. Like a home faucet at 1/2 to 3/4 of the way open. Weigh the hose down so that the water flows over the hole you will be cutting (the circle you drew earlier). This is important because the water it pulling away the heat that may cause the glass from the aquarium to crack, as well as the dust from the glass.

As you drill the tank, I have found starting at a slight angle helps score the glass enough to grip. If you go straight downward, the smooth glass causes the drill to move around. Once you have a slight divot, you can straighten the bit slowly to get a good, 90 degree cut. Do not push on the drill like you would if you were drilling into wood. Let the weight of the drill carry the hole being cut, adding only mild force as needed. Too much force can cause the glass to crack.

Remember to GO SLOW. Especially near the end of the cut. This will reduce the amount of chipping of the glass.

Tips when you Drill a Glass Aquarium


The main tip is to GO SLOW. I keep saying this because you WILL get bored and irritated, but don't rush it or you will regret it.

Hole Saw Guide

One of the BEST tips I can give you is to spend the extra $20 to get a Glass Hole Saw Cutting Guide. This will help hold the hole cutter bit and prevent wobbling, giving you a more accurate cut!

Use Tape

Another tip I have heard, but not tried, to avoid chipping is to put tape on the inside glass of the aquarium where you are drilling. I haven't tried this myself, but hear good things.

Putty Ring

To reduce heat, you can make a ring of Plumber's Putty roughly 1" from all sides of the hole you're cutting. This will help keep water on the hole you are cutting and moving heat away better.

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Hopefully this article gave you more confidence in learning how to drill a glass aquarium. The key to success is patience, planning, and keeping good water flow, removing heat. The first time drilling a glass aquarium is usually very scary. I recommend practicing on some cheap 5 or 10 gallon tanks before trying to drill an expensive aquarium. With these tips and instructions, I'm confident you will be successful in drilling your glass aquarium.

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About the Author

Reef Stable Founder John Krenzer

John is a Software Engineer with a passion for saltwater aquariums, as well as the founder and president of Reef Stable. He started in the aquarium hobby as a child with a 20 gallon freshwater aquarium. His interest in aquarium life grew and in 2008, John set up his first saltwater aquarium.

Today, John maintains an over 300 gallon reef tank system, consisting of a 120g reef and a 210g reef. These large tanks are contained within the same system, sharing a sump as a means to reduce total maintenance and increase total water volume.

John writes articles for the blog as a means to learn about more reef aquarium topics. These articles act as a reference for the readers as well as himself. John updates these articles frequently to provide additional information or make corrections as new information becomes available.

If you would like to request an article, tank tour article, or to collaborate, let me know via the Contact Me Page!

About Reef Stable

Reef Stable was initially founded in 2019 as a reef tank parameter log to fill a need. Reef Stable quickly grew, becoming a location to solve all of your reef tank problems as well as a place to learn.

Reef Stable now provides a Reef Blog, Reef Aquarium Guides, Coral Care Guides, Identification and Solutions for Pests and Algae, and Reef Dosing Calculators, in addition to the original Reef Parameter Log.

Reef Stable continues to grow, striving to provide a single location for all your reef tank needs!

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