Sumps In Marine Aquaria

Often we see people talk about sumps when it comes to saltwater aquariums. What is a sump? In short – it’s an additional tank, typically underneath, your display tank. We’ll discuss how to run a sump, why you should, how to make your own, and the benefits it can provide!

What is a sump?

A sump, as we discussed above, is just another tank or container that you add on to your existing tank. Simply put – it’s a place to house your equipment so that your display tank looks nicer and less crowded. It can also open the amount of equipment you use as well as the different types available for in-sump applications. It’s a great tool for especially saltwater tanks of all types.

Why have a sump?

There are many benefits to running a sump. Increased water volume is the first, most notable, feature here. By increasing your system volume you can help keep water chemistry more stable and dilute byproducts such as nitrates easier. Another benefit is that you could put all your extra equipment down there to hide it from view. Having equipment like a heater and protein skimmer hang out in your tank can be a real eye sore and distract form the natural beauty that is inside the tank. Third, it can provide extra flow into your aquarium with the use of a return pump. This pumps water back into the tank from your sump below. Fourth, it keeps the surface of the tank clean and clear with the use of an overflow box. This skims the surface of the water and as water tumbles down together it mixes and allows itself to be filtered out through something like a protein skimmer or filter sock. Finally, the last major benefit is the option of extra add-ons to your tank that just aren’t possible without a sump. Filter socks are one example and refugiums are another. You can also get much better quality of protein skimmer that are designed for in-sump use only.

How to set up a sump

Setting up a sump is relatively easy but may seem complicated your first time. Basically you need another container under your tank. Finding a used aquarium on something like Craigslist is a great way to save money here, or you can buy a premade sump from websites like Bulk Reef Supply. We’ll split the rest of this between tanks that are “reef ready” or drilled, and tanks that are not drilled with holes at all.

Drilled tanks – These are pretty easy to set up since they have a hole drilled in the tank, either in the back of the tank or in the bottom. Most even come with an overflow box and bulkhead already attached and ready to use! All you normally have to do is connect pvc pipe or flexible hose to the bulkhead down to your sump and then run a different tube going from the return up to your display.

Non-drilled tanks – Still easy to set up just different steps. If you don’t want to drill your tank, for fear of cracking the glass, you can use a simple hang on overflow box. Just like it sounds, it hangs onto the back of your glass tank and lets water over the top of the tank with a small U shaped tube. Then you connect your plumbing like normal.

Making your own sump

Making your own can be quite a bit cheaper than buying a premade sump. Using an used aquarium you can insert baffles into it. Lowes sells thin panes of glass or acrylic and they can cut them to the size you need. Just silicone in some baffles, let it dry and you’re done! Baffles are helpful for a few reasons. They trap air bubbles in certain areas so they don’t get into the return pumps. They keep parts of the sump with stable water levels while the return area is usually the area that has variable levels. It’s easy to mark your glass with a marker at the fill mark and add your top off water until you reach the mark again. Baffles can also seperate the areas in your tank like a protein skimmer chamber vs the refugium chamber.

Making your own has another big advantage to it. You get to design it the way you want to use it! You can make the refugium chamber as large or small as you want for example. With premade sumps you just have to take what they give you.

Sump designs

Here I’ll show two diagrams of common sump designs. It’s considered common for a sump to have 3 chambers. What you do with those three chamber is up to you but here are some common ways people use them.

Sump design with return in the middle of the sump

Here is a common one. Water comes into chamber one where it goes past an over/under baffle system to remove air bubbles. Then it moves into chamber 2 where the return pump is. The return pump has a T connection that lets some water go back to the display tank with some water also going towards the refugium. The advantage of doing it this way is that you can control the flow into your refugium making it a lower flow than the rest of the sump.

Sump design with the return in chamber 3

Here is the other common set up. Water flows into chamber 1, through an over/under baffle system into chamber 2. From there it goes over another baffle to the return pump in chamber 3. This set up is more common if you would rather run equipment instead of a refugium, maybe something like a calcium reactor or biopellets instead. This is also good if you want to run an algae turf scrubber which we’ll discuss in the next article.


The plumbing of a sump is quite simple although it can look complicated to someone new. Basically you have a bulkhead that goes on the hole in the tank (or overflow box if you don’t have a drilled tank). You simply connect PVC pipe, or flexible tubing, to the bulkhead and run it as straight as you can into the sump. This is why most sumps are underneath the main tank; to give a straight shot down into the sump. Wherever your return pump is doesn’t matter for this part. Just connect the PVC pipe, or again flexible tubing if you choose, to the return pump and make it as straight as possible up to the display tank. It’s quite important here that you use as few bends and turns as possible to reduce the head pressure of the return pump. If your tank is drilled there is usually an existing bulkhead for a return. If it’s not drilled just run your PVC over the top of the aquarium rim and back down into the water. A couple of 90 degree fittings works well for this purpose.

Your return pipe should be as close to the surface of the water as you can. The reason being is that if the pump gets turned off, your return pump will siphon water back into the sump until air breaks the tension. By having it close to the surface only a small amount of water, if any, will go down into your sump.

Pipe sizes for the return and overflow should be the same or close in size. I run both of my pipes as 1″ tubing going down and up. Some people like to run a smaller size for the return but my return pump feeds two different pipes so the flow is spread out more. If one pipe is bigger than the other, make sure the bigger pipe is always for the overflow box. If the return is smaller that’s ok, as it will put less water into the tank than the overflow can handle. However, if the overflow has the smaller pipe you can add more water to the tank than it can drain, leading to a giant mess waiting to happen.

Testing the sump

Now that you have your sump made, plumbed, and ready to go you might be wondering how much water do I need to add to the sump? I started by filling my sump to the top of the baffles but there isn’t an exact science here. We’ll have to adjust so be prepared to not only add more water if needed but also have equipment on hand to remove water as well.

Turn the return pump on now. Watch the return chamber and make sure it doesn’t get so low that the pump sucks air. If it does get too low just add a gallon of water at a time. Watch it for a few minutes while the water in the system equalizes out. If it works well and remains stable, you can top the water off to the desired level in the sump. Be sure to leave enough room for a few gallons of water to still enter the sump in our next step.

The emergency test, as I like to call it, is VERY important to do when setting up a sump. To do the test is simple; pull the plug on your return pump to shut it off suddenly. Watch your sump very carefully now and the return pipe will likely siphon water back down into your sump. If it starts to get too full, begin removing water immediately. The point of this test is to make sure that in the event of a power outage your sump will be able to handle the water from the display aquarium. This is the exact reason we place our return pipe as close to the water surface as possible. In my system my return actually sticks above the water line about halfway so it sucks barely any water into the sump.

Now that you made your your sump can handle all the water during a power outage, plug it back in. It will take a few minutes before the system equalizes out again but now you want to make a mark, using a sharpie or other marker, on the water line in your return chamber. This is your way of knowing how much water to top off the tank with. Simple add top off water each day until you reach this mark and you’re done!

This is a great reason why people say you should stick the biggest sump you can under your tank. For instance I am running a 55 gallon sump. With the baffles it holds roughly 30 gallons of water. This gives me an extra 25 gallons of emergency room in my sump to hold extra water in the event of a power outage. If I was using a 20 gallon long tank and the baffles held 15 gallons, I would only have 5 gallons of safe room left.


I hope this article helped explain what a sump is and why you should have one. If you have any questions about sumps please comment below and I’ll get back to you!


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