What is in this guide?
- Test fitting equipment and design with a mockup
- Choosing the right thickness of glass
- Where to find glass for cheap
- Getting exact measurements for your cuts
- Cutting the baffles and dulling the sharp edges
- Silicone prep work – cleaning and taping
- Clamping and supporting the baffles during glue up
- Silicone application techniques
DIY sumps don’t have to be sloppy
You may have seen some sump building videos on YouTube. There are quite a few. Many of the the people that make them seem to care about showing how fast they can do it, rather than doing a good job. They often end up with silicone gobbed all over the place or with baffles that leak and have to be redone anyway.
One thing you often hear with sumps are that “small leaks aren’t a huge deal” and that sort of thing. That isn’t really true. If you have an unintended leak between baffles or dividers then it can cause problems. These problems may take a while to show themselves, say with an ATO sensor. It’s best just to do a good job and leak test the sump as you would any other part of your system.
While some people don’t care too much about how the sump looks, some do. This is proved by how many expensive pre-made acrylic sumps are sold every year. While it’s a little more work, it is possible to build a decent looking sump yourself, and this guide goes over the methods to make it happen.
Silicone Injection Method
The methods in this guide are some of the same ones used by professional tank builders. First, the tank and panels are taped and prepped. After that, the baffles are bolstered into position. This is usually done with clamps on the edge of the tank holding small blocks of wood. Next, silicone is injected into the gaps. After smoothing, the result is a continuous bubble-free seam. Applying the silicone in one quick process prevents it from “skinning over.” This is when the outer layer begins the curing process and begins to toughen.
Professionals all have their own tricks of the trade, but this guide gives an outline of a way to copy them. It is not the only way to do it, but it results in far less air bubbles than other methods I have tried.
Once you have planned out your sump and know the rough dimensions, you will want to come up with your final dimensions for each piece.
Getting final dimensions for Baffles and Dividers
How to Measure – Allow about 1/16″ (1.5mm) to 1/8″ (3mm) gaps where the baffles touch the walls of the tank. This allows you to inject the silicone so it flows in the gap and creates a continuous seal between the two pieces. A gap closer to 1/8″ (3mm) allows the silicone to flow easier into the gap, if it’s too narrow, it becomes harder to inject in the opening.
For example, if you are measuring for a full-width panel that spans the tank front to back, here is what to do.
Say the inside measurement of the tank is 10″ wide. You would minus the 1/8″ gap from each side, adding up to 1/4″. Subtract that from the measurement and the resulting panel needed would be 9-3/4″ wide.
Apart from creating a good seal, the gap is left to avoid having to force or wedge a panel to fit which you should never do. While it doesn’t have to be too exact, it is better to have it a little loose than too tight. The silicone will fill gaps up to about 3/8″.
Spacers of the proper thickness will come in handy for this. Some people use zip ties or small pieces of plastic. In this build I used some rigid straws. The spacers on the sides can be removed as you fill the gaps with silicone. The spacers on the bottom should stay for about 45 minutes to an hour to prevent the weight of the panel from squeezing the silicone out from the bottom seal. You may also want to affix very small spacers to the bottom of the piece that will be permanently in there.
Planning your assembly
Once you calculate your final measurements, it’s helpful to do a mockup with cardboard. Just cut the pieces with a box cutter and tape them in the aquarium. This little extra step allows you to check a few things and identify any spacing problems with things like your skimmer or other equipment that might not be as evident on paper.
It might seem unnecessary, but this does a couple things for you.
- Once you cut a piece of glass, you can’t really just shave a quarter inch off it if you make a mistake. You get one shot or you must start over using a new piece.
- It’s nice to lay out the footprint of where everything is going to go. You might discover a piece of equipment won’t squeeze into a chamber like you intended. You’ll also find out what areas will be hard to access with your hands (say for cleaning or netting out fish).
Once you do this test fitting and you know how big to make the panels, you can either cut them yourself or take your order to a local glass shop.
Selecting the Right Glass Thickness
You will need your baffles to have some strength.
Strong panels will stand up to being knocked around a little. When you are trying to pull out your skimmer, you are usually hunched over the sump and you aren’t thinking about being careful.
Aside from not breaking easily, thicker glass prevents flexing or bowing from water pressure. A broken baffle in a full sump is no fun. It means draining, drying, scraping, and all of the hassle that comes from having your sump offline for enough time to re-cure new silicone.
Using 1/4″ (6mm) glass is what you should aim for for baffles. Quarter inch glass is also sometimes called plate glass. If you can’t find glass this thick, you might resort to something around 3/16″ (4.5mm), but that’s about the minimim.
1/8″ (3mm) glass is easy to find at home improvement stores. It is too brittle and you should avoid it.
A good source of glass is from an old tank. You can buy one for cheap. There are a lot of tanks on Craigslist or in second hand stores.
If you are looking at taking the glass from an old aquarium, start looking at tanks sized 29-35 gallons and up. Glass of at least 3/16″ (4.5mm) is found on these tanks.
What about Acrylic Baffles?
Some people use acrylic baffles in their sump. This is often done because it is a little easier to cut and work with.
In a glass sump, you should ideally use glass baffles. Using silicone to affix acrylic panels to glass doesn’t work well in general because it can only form a very weak bond. Bulk Reef Supply actually did a good experiment to show glass vs. acrylic silicone bonding. For a strong permanent bond, glass panels work best. It is also nice to be able to use a razor blade to scrape them after several months of use.
What type of Silicone?
Use a brand of silicone that is right for the job. Whatever brand you choose, you must ensure that the one you choose does not have any mildew-resisting chemicals in it such as brands made for use bathrooms and kitchens. There are a lot of types of silicone available at your local hardware store but they are all low grade sealants rather than adhesives. The easiest thing to do is just to get the good stuff from Amazon instead of trying to save $5. A good brand is “GE SCS1200” which has very good strength and about 15 minutes of working time. It is often recommended by professional tank builders and what is used in the build shown in this post. Another brand that gets recommended a lot is “Momentive RTV103”.
A single tube of silicone should be enough for 4-5 baffles. On this 40 gal sump I used 3/4 of a tube.
Cutting Glass Pieces
Using a standard glass cutting tool and a straight edge, the piece should be scored with a nice clean motion. Never go over the same score twice as it will dull the cutting wheel. It is worth it to do a little research on YouTube on the proper technique for this, but I won’t get into it here.
Roughing and shaping the panels
The edges of a newly-cut piece of glass can be extremely sharp. Edges should be ground with a sharpening stone or sanding block just to take off that sharp edge. Dulled edges will make it safe to reach in the sump with your hands and arms. If you are having a glass shop cut your panels, they can polish the edges but you really just need to dull them down. Wear gloves when sanding or grinding the glass.
Panels that sit on the bottom will rest on the silicone of the existing tank. This something we need to be careful of because the newly cut piece may have very sharp corners. It’s a good idea to nip the corners and sand them with the sharpening stone just so you don’t mess with the seal on the existing tank too much.
Cleaning the Glass
After cutting and sanding the edges, it’s time for prep on the pieces. You can start with wiping down the pieces with just water and some paper towel to clean them up a bit. Best not to use Windex or glass cleaner as it contains ammonia.
Parts of the glass that will be covered with silicone should be scraped with a razor blade and cleaned with Acetone (nail polish remover) or Rubbing Alcohol on a paper towel. It’s a good idea to take your time with this step and do a good job. This will remove any old silicone and clear off fingerprints for the best possible bond.
Installing Panels and Baffles in a Sump
In the sump design post I show water moving from one end of the tank to the other, flowing from chamber to chamber like a waterfall. This is done to make the concept simple to explain and show.
Most designs just have rectangular baffles that span the width of the tank. It’s easy to design and make them that way. While you can’t really form glass into whatever shape you want, you aren’t just stuck with rectangles either.
There is a lot more you can do with a sump when you realize this.
Creating openings for water to pass through makes it so you don’t need too many baffles. Fewer baffles means you save space. Cutting a notch in the corner of a baffle is one technique you can do, and this is how to do it.
Cutting a Notch in a Corner of a Glass Panel
This method shows how to create a corner notch in a glass baffle. First drill a hole, then make relief cuts to remove the material you want. More info about how to drill glass is on this page: Drilling Holes in Glass Aquarium Tanks
Here is a video showing cutting a corner notch with the same technique but with better tools and results. It is easier to get a good clean hole without chip-out with thicker glass.
Securing Panels and Final Preparations
In this example I am showing the process I followed on a brand new 40 breeder. I got the glass for the dividers from an older 35 gallon that I cut apart (3/16″ or 4.5mm thickness).
It is handy to have some scrap wood to help secure the pieces as you work on them. Wood blocks secured with clamps will help to hold the panel you are working on. Spacers placed around the edges will help to create the proper gap (1/8″ or 3mm) around the piece.
Taking a little more time to secure the panels with spacers and wood blocks might add time to each step, but you’ll be glad you did. Silicone that has high strength has less working time than you might be used to, and you want to apply it in a quick deliberate process.
Smoothing and Removing Tape
Taking time to prep each seam will result in a much nicer end result. Each panel can be taped off along with the edges of the tank (as shown). After applying the silicone and smoothing it with your tool, remove the tape right away while the silicone is still wet.
During the few minutes of time when you are applying the silicone, you will learn a lot. Your preparation with taping and securing the glass panels will be evident.
The amount of silicone needed can vary. It depends on how you measured and cut your panels. For example, if you made the gaps between the existing tank and the edges of the panels larger, then more silicone will be needed. The target is about 3mm or 1/8″, so with a larger gap than that, you will be pumping the gun more.
If you only left a very small gap to fill (say less than the target of 3mm or 1/8″), less silicone is required. This may make it more difficult to get a good seam however, because the tube’s tip will have to be pressed firmer into the joint as you squeeze it out. This might make you work slower, and the silicone will get tacky as you use up the working time.
The way you cut the tip of the tube can also affect the process a lot. Some people cut the opening at an angle and then clench it with pliers to make it flatter. A lot of it is personal preference.
If you are working with a panel that is close to another such as a bubble trap, then you’ll only be able to apply the silicone from one side of the joint. In this case, you want the silicone to easily flow through the joint when it’s squeezed in from only one side. A proper sized gap (closer to 3mm or 1/8″) would be helpful in this situation to get the silicone to create a continuous seam.
Applying silicone needs to be done swiftly to get nice looking results. There are too many techniques and concepts to describe fully, and tank building is a skilled art. Experience will teach you more than reading tips and tricks ever will.
It is very important to let the silicone cure for at least 7 days. 2 weeks if possible. This is done to protect your system from the effects of off gassing in the curing process.
You will discover many tips and tricks of your own as you work. Your 2nd sump with probably turn out better than your first. You will probably realize this is one trade where experience really counts.