Freshwater Sump Filter | Convenience, Planning & Designing

A sump filter is frequently associated with marine or saltwater aquariums. But it’s also perfect for Freshwater Aquariums, especially if you want a big tank!

This type might just do the trick for those thinking about getting an extra-large fishkeeping setup!

A Freshwater Sump Filter may hold more media than a Canister or HOB Filter.

What Is An Aquarium Sump Filter

It’s just a tank where you can put your filtration system, media, pumps, and heater so that you don’t have to worry about cluttering up your display tank. 

Having a large sump filter is advantageous because it can separate more dirt and contain beneficial bacteria. Freshwater aquarium sump filters are an excellent option if you’re planning on getting a large tank.

The materials needed to construct an aquarium, such as glass or acrylic, are also used to build an aquarium sump filter. Sump filters can be placed underneath the aquarium. When water quality is critical, sump filters have traditionally been employed. 

An aquarium sump filter’s outstanding, effective, and efficient filtration appeals to the freshwater fishkeeping hobby.

Why Should You Use A Freshwater Sump Filter

The sump’s primary function is to keep the display tank isolated from the aquarium’s technical components. The sump is a separate tank beneath the display tank that serves as a backup for the main tank.

Sump systems allow you to use large amounts of media to improve your filtration. Biological filtration is the primary concern we refer to when talking about filtration. 

The main benefit of installing a sump system is the increased volume of water available to the system. As a result, it is more stable and less susceptible to changes in pH. 

In addition to allowing you to use a more extensive filter system, the sump can be filled with temperature sensors, UV lights, and other devices.

The following are the key advantages of a sump:

More Flexibility

Adding a sump filter to your aquarium system allows you to use media such as ceramic rings, foam beads, and fluidized moving beds. 

It is fully customizable as per your requirement and choice. And there is a lot of room for filter media here.

In addition, various attachments, such as heaters, sterilizers, and other apparatus, can be accommodated hidden from the main tank’s visible surface.

More Volume

Some filters can only handle a specific volume of water. On the other hand, aquarium sump systems can be built of any size.

So it increases the filtration capacity in terms of the volume of water in your aquarium system, and you can have a larger tank.

More Stocking

More filtration and volume will be available when installing an aquarium sump filter. Therefore, it is possible to overstock the sump-connected tank without harming the fish.

More Stability

An aquarium sump filter is much easier to maintain than other filter systems, making it easier to manage water quality in a larger tank. As a result, it helps maintain a consistent water quality level.

More Aesthetics

The main display tank will have no equipment, so your fish tank appears clean. Thus it is aesthetically appealing and provides room for aquascaping.

More Versatility

You can also grow moss, delicate plants, or shrimp in a refugium, installing it inside the sump.

A Freshwater Sump’s Design Consideration

Glass or acrylic must be used to make an aquarium, and sump filters are no exception. Underneath the aquarium, sump filters are placed. When water quality is critical, sump filters have traditionally been employed. 

However, freshwater tanks are growing more significant these days. Therefore, aquarium sump filters are becoming increasingly popular in freshwater fishkeeping because of their outstanding, effective, and efficient filtration. Design considerations for freshwater sump tanks are outlined here.

1. Calculate The Sump Flow Rate:

A sump’s flow rate should be six times as high as the tank’s volume in gallons per hour. For instance, if your tank has a capacity of 100 gallons, a return pump of about 600 Gallons per hour should be installed in the sump.

Flow rate of Sump Filter

Your tank’s water will cycle through the sump six times each hour if you do it this way. Therefore, the return pump flow rate should be considered when designing your sump and piping.

2. The Size Of The Sump:

There will be more space for filter media and other accessories if you have a larger sump. You should aim for the largest size to fit in your stand when it comes down to it.

However, you should keep your sump at least 20% of the volume of your tank. A 20-gallon sump should be plenty for a 100-gallon tank. 

You must also consider the amount of water that will be drained to the sump tank when the return pump is switched off or if there is a power outage.

Sump Volume

For a straightforward approach, divide the gallon by the tank’s height! In our example, the tank is 100 gallons in volume and 20 inches in height. 

Consequently, we anticipate that around 5 liters of water will be drained from the tank. So rather than risk flooding our room, we’ll double that amount, which is 10 gallons. 

So by this approach, If you choose a minimum sump size of 20%, add a 10-gallon addition for a minimum sump size of 30-gallon. If you want a 30 percent sump, your sump size will be 40 gallons.

Adding water volume has now been clarified if you weren’t clear before. So even though your main display tank holds just 100 gallons of water, your fish have access to at least 130 gallons of water!

3. Sump Dimensions:

Consider our tank as an example. Assume its dimensions are 52 inches by 22 inches by 20 inches and that it has a standard stand. 

Because we are constrained by the dimensions of our stand, we must take measurements of its inside.

There should be a 4 to 6 inches gap between the sump and the tank stand to comfortably perform sump maintenance. So the inside frame should have a 45X18X15 dimension.

Freshwater Sump Dimension

As a result, your sump cannot be taller than 15 inches in our example. We can hypothetically put a sump of 50 gallons (45X18X15 cubic inch = 52 gallons) inside the stand with this information in mind!

Nevertheless, you may not want to construct the most oversized sump possible in the stand. As a result, this article will discuss calculating the sump’s minimum size for a 30-gallon tank.

One thing to remember is that even when the pump is turned off, and the water is draining to the sump, we still need at least 1 inch of safety clearance. So now that the tank’s width is 17 inches and the height is 14 inches, have been determined.

We can experiment with the tank volume calculator to get the exact length required to hold 30 gallons of water. And you will find that if you have a length of 29″, then u have ~30-gallon. 

What Media To Put In A Freshwater Sump

Freshwater Sump Filter Design

As far as aquarium filtration systems go, the sump is far better than any system. There is no upper limit to the amount of media added to a sump filter.

You can add the following media to your sump, but remember to arrange them in the correct order.

Sump filters and other aquarium filters should be arranged in this way:

1) Mechanical Filter Media:

Mechanical filter media are – filter socks, filter floss, and pre-filter.

2) Biological Filter Media:

A few examples include bio-balls, k1 media, ceramics rings, lava rocks, and more.

3) Chemical Filter Media:

Activated carbon is the most prevalent filter material.

Basic Freshwater Sump Design

A sump for a freshwater aquarium is made up of three primary components:

  1. Mechanical Filtration Chamber
  2. Biological Filtration Chamber and
  3. Return Chamber

The primary sump design for our 29″ x 17″ x 14″ sump is shown below. The first, second, and third baffles are all 10″. The size of the fourth baffle is 9″. 

You can notice that baffles 1 and 3 are 1.5″ below the top. This is required as a safety precaution as if one chamber becomes clogged for whatever reason, and the water will overflow into the next chamber rather than flooding the room.

Baffles 1 and 3 likewise have a 2.5″ gap from the bottom. This is how the sump will function. Water from the overflowing tank enters the mechanical filtration chamber through the drain lines.

Freshwater Sump Filter Planning

Mechanical Filtration Chamber

Typical scrubbers for dishwashing will be the first media for the mechanical chamber. Synthetic cotton will follow.

Because dishwashing scrubbers remove coarse particles and synthetic cotton removes fine ones from the water, we employed two different media for mechanical filtration.

The scrubbers are washable and can be reused with just gentle rinsing during routine maintenance. In addition, compared to natural cotton, synthetic cotton is significantly less expensive. 

Because it is so inexpensive, it is preferable to just dispose of the old media and replace it with the new. However, you can reuse the cotton after rinsing and cleaning it.

Biological Filtration Chamber

Once the water has been mechanically filtered, it will go to the biological filtration chamber via the first set of baffles. There should be as many ceramic rings in this chamber as they can contain. 

Ceramic rings have a larger surface area than bio balls; hence they may support a more significant number of beneficial bacteria. Ceramic rings, on the other hand, are much easier to clean. The old tank water can be used to clean the tank during routine maintenance. 

But bio balls require more shaking to clean, and you’re probably also removing some good bacteria when you do so.

tray for biomedia - freshwater sump

The media will be held in place by a tray-like structure, allowing water to flow through it. The plastic paper trays sold at stationery stores can be chopped into smaller pieces and used as paper trays.

Remember that our goal is to keep the media in place while allowing water to flow. Therefore, egg crates are also a preferred method. 

A tray-like device can be used to raise the media up from the sump’s bottom, preventing the accumulation of trash in the biological chamber.

The mechanical filter will catch any debris or dead bacteria that make it through to the main tank.

Return Chamber

Let’s return to how our sump works. After passing through the ceramic rings, the water is biologically filtered and ready to return to your tank.

As a result, the water enters the return chamber after passing through the final baffle. This is the location of your return pump. If you have a heater, it should be placed in this room.

The water is routed upwards after passing through mechanical filtration (top-left side of the biological filtration chamber) and then driven downwards (bottom-right side of the biological filtration chamber).

Remember, if water just passes over the biological filtration chamber, it is not filtered, as water will always choose the path of least resistance.

Finally, the final baffle differs between freshwater and marine applications. We’re making the water descend from the top to the return chamber here. This aids in the oxygenation of the water before it is returned to the pump.

Water should be channeled from the bottom to the return chamber in a marine sump so that air bubbles do not enter the main tank. So proceed with caution.

Another thing to look for is at least a 2.5″ gap between the baffles to get your hand in there and clean as necessary. If the spacing is too small, cleaning will be difficult or impossible.

If activated carbon is required for chemical filtration, simply place it between baffled 3 and 4, and the water will be chemically filtered before entering the return chamber. When it is no longer required, simply remove it from the area.

freshwater sump example
Image Source:

Testing Sump Filter

To determine if your sump filtration system has a problem, you must do a sump test.

Testing procedures:

  • Complete the piping for the sump filtration system and connect it to the main aquarium.
  • Fill the sump halfway with tap water; when the tank is full, it will start dumping water into the main tank.
  • When the sump is filled two inches below the surface, stop filling and let the main tank drain the excess water.
  • Make a note of the water level in the sump. This will be the maximum water level in the sump when the power goes off, ensuring that the sump does not overflow.
  • Turn on all sump pumps and skimmers. The water level should have fallen by one to two inches, depending on the size of your sump filter system and the capacity of your pump to pump water.
  • As the water level in your sump decreases, be on the lookout for leaks. Because of this, the water level won’t change. Red stickers indicate that the water level is three inches above the pump. When the sump filter is functioning, this is the water level that should be maintained. Add water if the level falls below three inches.

Evaporation regularly lowers the water level in the sump due to the open design. To keep the water level at the marked point, you must replace the sump whenever it lowers by one inch below the line.

Instead of absorbing air, becoming exposed to the air, and causing the filter to overheat, this might damage the filter.


Is It Better To Have A Sump Or A Canister Filter?

As opposed to any other aquarium filter, a sump filter is preferable, in our opinion, because it is more adaptable and can contain more media than a canister filter. Most crucially, a sump filter may provide the same level of filtering for half the expense as a canister filter.

Is It Okay If I Keep Fish In My Sump?

The sump is designed to hold aquarium equipment and filter media; it will no more be a sump if you wish to keep fish in it. Instead, it’s just a tiny aquarium beneath your main aquarium.

What Is The Procedure For Adjusting The Water Level In Your Sump Filter

A ready-made sump will have an “overflow level” marked on it, and your water should be at this level. If you built your own sump, the water should be at least 1.5 inches below the tank’s top. 

Because of this, the first partition baffles should not be breached by water levels. Only the baffles should be able to supply water to the rest of the system.

Is There Any Suitable Amount Of Water In A Sump?

The sump filter should never be filled to the brim. Because the sump must be able to hold the tank’s overflow water when the power goes out. 


This type of filter is frequently associated with saltwater setups, but it can also work great for freshwater tanks – especially if you want to hold more media than you could with a traditional canister or power filter. 

If you have any questions about setting one up, don’t hesitate to leave us a comment below. We love talking about fishkeeping and would be happy to help out however we can!

Sujit Modak

Leave a Comment