The world beneath...some thoughts on freshwater sumps...

am a big believer in sumps.

Yes sumps.

"Seriously, Fellman....You're busy spouting reef-speak again!"

I know, you have visions of an absurdly-complicated reef system, with every possible gadget attached to the tank, costing thousands of dollars/euro/pounds, etc., while yielding only marginal performance benefits over more "conventional" freshwater filtration systems, right? 

Yeah, I hear that a lot.

First off, don't think of a sump as a "filter" in the conventional sense. Think of it as a sort of "water management system" for your display. To call it a "filter" is way too simplistic, IMHO. A sump is literally the "nexus" of your water management system- a location to take care of many of the environmental management tasks in one convenient, discreetly hidden place! 

And it doesn't need to be a byzantine maze of complexity, either.

Now, sure, reefers like them because we tend to be "gear-centric", and sumps are an excellent place to house all of the complex stuff we use to maintain our systems. That being said, sumps are excellent ways to do stuff that we don't want to do in the display. Hue? Read on...we'll get to it.

Although it can accomplish a lot of complex tasks, a sump need NOT be complex. In our context, a sump could be defined as virtually any type of container used beneath or behind an aquarium. It holds water and provides a location to place various pieces of equipment that our systems need (Yeah, even a setup consisting of a simple spare 10-gallon aquarium set up below your 50 gallon display tank to receive and process water is a "sump" by this definition).

Now, coming from a reef aquarium background, where sumps are simply the way 99% of reef systems are set up, I'm sure that it doesn't surprise you that I like them. The need to accommodate ancillary support gear like protein skimmers, reactors, etc. is just one reason why sumps have evolved into the "hub" of most reef aquarium systems, as mentioned above.

Yet, the more I play with exotic ideas in my freshwater and brackish water systems, the more I'm realizing the value of the sump, and how they can benefit freshwater hobbyists as well. I'm always surprised to see high-end freshwater setups with canister filters and reactors and such instead of sumps. Seems sort of..well, "clunky" to me. (is that like a word? "Clunky?" Whatever, right?)

Sure, we see them in some Discus tanks, African Cichlid systems, and occasionally a planted tank, yet they are the exception, rather than the rule. I'd love to see their use more widespread in the general freshwater world. Now, I realize that the breeder who keeps a hundred 5 and 10 US-gallon aquariums is unlikely to want set up a sump for each one, and the idea of a central filtration system (either incorporating a sump or some other system) is fraught with potential issues, such as transmission of disease, etc.

However, I think sumps would be a good idea for most freshwater display aquariums. I think that even modest-sized aquariums (like 40 US gallons and up) could benefit from sumps. Now, yes, there is the issue of expense and additional design considerations...You're essentially adding another little aquarium. And of course, you need to have an overflow weir, which means a "reef ready" tank (unless you want to do some retrofitting and install an overflow). And a reliable submerged or external return pump, sized properly for the system.  Yeah, a bit more work, perhaps, than simply hooking up the old Eheim... 

Then again- dealing with glassware sort of sucks, IMHO! How many have YOU broken while cleaning them? Be honest...

The advantages of this extra effort are actually numerous.

For advanced concept or speciality display tanks (like our blackwater and brackish-water systems), sumps offer extraordinary flexibility and advantages over more traditional systems, like canister filters, sponges, and outside filters. I mean, the aesthetics alone are reason to consider such an hardware visible in the aquarium is always a plus in my book. A sump allows you to place the heater, reactors, or other tech equipment conveniently out of view, yet easily accessible for access and maintenance.

That's all well and good from an aesthetic standpoint, but what other reasons would there be to use a sump in one of our systems? What are some tangible benefits? Well, to start with, I like them because they add water volume to your overall system, A typical sump (in the reef aquarium world) is anywhere from 20% to 50% or more of the volume of the display itself. And of course, this is good for a real "plus" that you can't help but consider: A sump adds volume to your overall system; volume means additional stability and biological capacity for your display. And a sump can act as a built-in "hedge" for evaporation. Sumps also facilitate Increased oxygenation. As water drains into the sump, air mixes with it, allowing for beneficial gas exchange, releasing CO2 and adding fresh O2.

(My friend Marc Levanson builds awesome custom sumps and has a great website filled with info on them..check it out)

Sumps allow you the flexibility to utilize different types of (filter) media, like botanicals/and leaves in our case, than for whatever reason, you might not want in the display tank. 

Leaves, in particular, with their associated decomposition, biofilms, and aesthetic considerations may be something that you simply don't want in your aquarium...but you might like the affect they have on the aquarium, in terms of environmental stability, cultivation of biological filtration, supplemental food sources, etc. Or maybe you want to play with live plants and not have botanicals "in situ"- or perhaps you want a "clinical" bare bottom Discus or other "concept" tank, but appreciate the "support" a sump could provide.

(I mean, you CAN really go crazy with all sorts of media in a sump if you WANT to..)

You could place botanicals, like Alder Cones, etc. into a reactor, which can conveniently be located in the sump.

And of course, with a sump, you can build in sections for the cultivation of food animals (like Daphnia or worms, etc.), creating, in effect a place in which to grow them free of predators (your fishes), feeding off of excess food and processing nutrients, with the occasional specimen getting pulled into the main display to provide a surprise "treat" for your fishes. Essentially the freshwater version of the "refugium" concept popularized by reefers years ago!

You could even light a section of the sump (on a "reverse" schedule of the display) with an inexpensive LED light to cultivate fast-growing floating or rooted bunch plants (like Water Sprite, Rotala, Hornwort, etc.) to provide pH stability for sensitive fishes, or assist with nutrient export via harvesting them. Oh, and a great "hack" for those who love nice aquatic plants but also happen to keep disruptive fishes i the display (like digging cichlids, vegetarian fishes, etc.).

These are just some of the most prominent and beneficial reasons for considering a sump for your next display aquarium. There are lots of other ideas you could play with...

Sure, you could adapt a canister to perform some of the functions (like holding "media"), or use a hang-on power filter as a sort of "moss reactor" or what not, but the concept of a sump, with it's spacious capacity and inherit flexibility gives you options and ease of operation that these "band aids" simply can't match. The ability to experiment with different "media" outside of the display is reason alone to consider one.

Even off-the-shelf "all-in-one" aquariums, with built-in rear compartments, essentially function like sumps, providing most of the same benefits as the remote units do (multiple chambers, extra water capacity, etc.).. And the possibilities are significant for these tanks, too! You can do all sorts of cool stuff with them!

The additional expense and planning that might be required when incorporating a sump into your next freshwater display will, in my opinion, easily be compensated for by the operational effectiveness and efficiencies you'll realize. 

All-in-all, sumps are a great way to give your system the "edge" it might need for long-term success and "mission flexibility" as your needs evolve or change. "kind of "future-proof", in most respects. Are they perfect for everyone? Absolutely not, as we discussed at the beginning of this piece. However, for many of us, they could open up exciting new possibilities for adventurous hobbyists with ambitious ideas...and that's kind of what we're all about, isn't it?

There's a lot going on in the world beneath our display tanks...

Stay open-minded. Stay creative. Stay innovative. Stay bold.

And Stay Wet.


Scott Fellman

Tannin Aquatics

Scott Fellman
Scott Fellman


12 Responses


August 12, 2022

Hi Scott,

You’re absolutely right about sumps, but you missed the single biggest advantage (other than a passing comment in the comments section) — dissolved oxygen and the role it plays in Co2 levels.

Planted tank folks want to crank Co2. Co2 suffocates fish. The higher the dissolved oxygen, the higher you can safely crank up Co2. Practically, the only way to increase oxygen in an aquarium is by increasing the surface are of the water… The suggestion usually put out in a fresh water context if through surface agitation. A weir does this loads better, along with all the other advantages of a sump.

One aquarist with access to the lab tools needed did some pretty hefty testing on this a long time ago, and found 3-5x higher oxygen levels with a large weir than with big canisters.

Scott Fellman
Scott Fellman

December 18, 2020

Hi Jim,

It’s a good question; one that comes up now and then. As a reefer, I can appreciate this, too! Now, the reality is that the typical protein skimmer as we think of it in saltwater doesn’t work in freshwater. A protein skimmer works with water surface tension, and freshwater has much lower tension. This makes sense if you observe airstone bubbles in fresh versus saltwater. The bubbles in saltwater are much much smaller than those in freshwater, and take longer to surface. Protein skimmers work based on “micro bubbles” (technically called “foam fractionation”) by picking up the proteins within the water column. The reality is that, unless you have a huge amount of dissolved organics, a protein skimmer really won’t work as effectively as it does in marine applications. There are koi breeders/fanciers who use a sort of foam fractionator process in their systems; do look for these online in YouTube and elsewhere. That being said, why NOT experiment? 😎`- Scott

Jim Yuen
Jim Yuen

December 17, 2020

Hey Scott,

Since we both come from saltwater backgrounds…Can a protein skimmer used for a saltwater aquarium work for a freshwater aquarium if additional air stones are added?

Thanks in advance,


Scott Fellman
Scott Fellman

March 25, 2020

Hi Chavez,

Your idea sounds just fine. The fist chamber should have a filter sock or some means of capturing larger debris before they hit the column of bioballs. Ideally, the filter sock or pad should be easily accessible/removable for cleaning or replacement. As far as the carbon is concerned, you probably want to place it in an area where the most water flow is coming in…Maybe even in a little pouch or filter bag either directly under the flow from the tank, or somewhere more “passively” in the sump. I do like the idea of substrate and plants I the center…You could even light them on a reverse schedule from the main display tank, to help even out the day/night pH fluctuations! We do that with reef tanks a lot. A good way to get the benefits of live plants in systems where the fishes would otherwise disturb them (think, digging fishes like cichlids). Hope this helps! -Scott


March 24, 2020

I recently bought an aquarium that was previously set up as a saltwater tank and it came with a really nice sump filter. I’ve only ever used hang on and canister filters. I’m trying to set up the sump as a freshwater and want to ensure I’m doing it right. My plan is to add bio rings, bio balls and charcoal to the side on the left with the sock. In the middle I would like to eventually add substrate and plants and on the right is where my pump is. Does this sound ok or even make sense?

Maarten Cappaert
Maarten Cappaert

May 14, 2019

Thanks Scott, that does help. I have a pretty nice water circulation using a cross flow pump. Sounds like the fish wouldn’t struggle with moderate flow so I will keep my configuration as is because it’s working so well.

Scott Fellman
Scott Fellman

May 14, 2019

Hi Maarten,

I actually use a pretty good amount of through in my sump-configured tanks. Although you tend to want a slower flow through the actual sump, in my experience, it’s useful to keep water movement in the tank moderate. Use of supplemental powerhead or submersible pumps can keep the water circulating at all levels of the tank. As you know, the water is drawn into the sump from the surface in a typical system (which is good for the gas-exchange at the air/water interface)… and detritus, botanical debris, etc. will typically settle out on or near the substrate- which is NOT a bad thing, IMHO (see our many articles on detritus for my thoughts!). The key is to get enough floe to mix the water well, without sending clouds of leaves and botanicals into the water column…a fine line that requires testing and tweaking! SO, the bottom line: Slow movement in the sump, moderate movement in the display!

Hope this helps!



May 12, 2019

Is there enough water flow in an Amazon biotope to pickup debris? I use a sump with overflow in my African cichlid tank but debris in a tank with lower flow rates tends to collect on the bottom where a filter intake has a better chance of picking it up. No? Thoughts? I am very curious about your experience because I am considering converting a cichlid tank into a South American biotope.

Scott Fellamn
Scott Fellamn

May 03, 2019

Hi John,

Think of a sump as sort of the “water processing center” for its host aquarium…Typically, a sump is located below the tank, where it receives water from an overflow. The sump can be equipped with mechanical filtration media (ie; pads, filter socks, etc.), chemical media, such as carbon, or even used as a biological filtration and settling area- You can put in quantities of botanicals, plants, etc. in a sump- the possibilities are endless…and a sump adds to the overall system capacity, too.

You are correct, that in the event of a power failure, the water will flow via gravity into the sump until it reaches the bottom of the overflow “teeth” or wire in the tank. So you need to size your sump carefully to accommodate the extra water that will flow down in the even of electrical interruption. You can use the old formula (LxWxH/231) to determine how many gallons per inch of tank volume there are- so you’ll know that, if the water drains, say, 3 inches to the bottom of the overflow teeth, that you’ll need to accommodate “x” number of additional gallons in the sump over it’s normal operating range in the event of power failure or whatever. This is why we in the reef community always suggest going with the largest sump that you can afford, and that your tank can accommodate- to be a sort of “insurance policy” for overflow! Hope this helps…lots of information on sumps and such is available in sites like Reef2Reef or Reef Central…


john arnold
john arnold

May 01, 2019

so i keep seeing people saying a sump is not a filter as well but the one above in this article is obviously a filter isnt it? Also im a bit confused if the pump stops doesnt the water keep flowing into the sump until the tank water level reaches the out flow hole in tank, i ask because my pipe for water to return to filter is only half way up my 4ft tank so i guess that would be too low yeah ?


Scott Fellman
Scott Fellman

April 11, 2019

Well- a sump pump is a different thing.

A “pump”, in this context, is referring to what is typically a submersible aquarium water pump. There are numerous, highly efficient models from which to choose, including DC pumps which provide fantastic power for little electrical consumption. Often, even more efficient than a typical aquarium power filter or powerhead!


Michelle Townsend
Michelle Townsend

April 10, 2019

The sump pump how is it on the electric bill?

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