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


Leave a comment