A Sump AND a Refugium?

Our aquarium systems grow over time. It’s almost inevitable. It’s well-nigh organic. And we surely all hope that as they grow, they get better. One proven way to dramatically improve the functionality of an existing system is with the addition of a sump or refugium.

The conventional overflow-with-sump/refugium plumbing design (we can call it a traditional design by now, could we not?) is well-tried; most seasoned aquarists would agree that it presents the overall most simple and effective scheme. To start, by drawing water mainly from the surface, the overflowing outlet keeps the air/water interface clear of floating debris, greasy slicks, etc. What’s more, sumps and refugia provide a safe and secure place to keep plants, animals and equipment that cannot be kept in the main (i.e. display) tank.

Sumps and refugia can indeed significantly improve the functionality of pretty much any aquarium system. Most notably, these benefits are compounded when a sump and a refugium are operated in tandem. To understand why this is so, one should consider the major functional differences between them.  

What is a sump?

According to the old Merriam-Webster, a sump is a “pit or reservoir serving as a drain or receptacle for liquids”—specifically, it is the “a pit at the lowest point in a circulating or drainage system.” By this definition, if we positioned our reservoir above the display tank (for whatever reason) and put the overflow chamber in it instead, so that it receives water from a pump within the display tank, could we correctly call it a “sump?” Or would that term apply most aptly to the display tank, as it is situated at the lowest point of the recirculating system? Since most of us keep our sumps below the main tank, we’ll leave the above question for the lexicologists to resolve.   

Seems a lot of planning, materials and work just to get some surface skimming, right? Maybe it would be. But there are some other huge advantages in using a sump system. Biggest among these is the way in which they allow for the consolidation and organization of many various pieces of standard aquarium hardware (heaters, protein skimmers, chemical filtration media, etc.). This makes actually using them a lot easier and also keeps unsightly equipment/plumbing/cable clutter out of the display. And don’t forget that sumps serve as a great spot to gently deliver dosed chemicals, top-off water, etc. to the system. In other words, they help to dilute and disperse additions that could otherwise disturb aquarium animals when administered in closer proximity.

So, by the above definition, isn’t a refugium just a type of sump? Maybe. But what makes a refugium a refugium has less to do with its purely mechanical function and more to do with ecology. Good old Wikipedia describes it well as “a marine, brackish, or freshwater fish tank that shares the same water supply,” separate but nevertheless “connected to the main show tank.” Most outstandingly, “it permits organisms to be maintained that would not survive in the main system” including certain macroalgae/plants, live foods and/or anaerobic denitrifying bacteria. Furthermore, water flow and lighting can be tailored to favor particular inhabitants.

Refugia are Grand Central for the exchange of water, particulate and dissolved substances and organisms in vital processes such as nutrient cycling/export, sediment deposition, plankton production, gas exchange, denitrification, etc. In some instances, one might even use a refugium to recuperate a stressed or injured animal.

For sure, the most common use of refugia is to concentrate and intensively cultivate photosynthetic organisms (especially macroalgae). The algae take up excess dissolved nutrients as they grow; nutrients are recycled when fed out to aquarium herbivores or is exported when disposed). The algae also provide ideal biogenic habitat for highly desirable microcrustaceans such as harpacticoid copepods. Because a dense bed of seaweed creates so much surface area, (and hence a lot of friction as water passes through it), water velocity gradients with a planted refugium can be intricately laid out and rather extreme. This causes suspended particulate matter to tend to settle onto the refugium bottom. Finally, planted refugia help to stabilize system water pH values when operated on a “reverse” or “alternate” light cycle.

Happy together

Some aquarists might have a sump. Some aquarists might have a refugium. Some have neither and may be wondering which would be best for their system. Not all know that there is good reason to install both! And of course, if you already have one, you can easily integrate the other.

The best placement depends on the system. For example, if you run a FOWLR (fish only with live rock) and rely heavily on biological filtration, it would be best to install a refugium downstream from the sump; in this manner, the water will be laden with nitrate and stripped of oxygen just before it enters the refugium from the sump (and biofilter). If you run a reef tank, you might want to use the opposite placement; in this manner, all of the yellowing compounds released by algae can be removed by chemical filtrants in the sump just before the water reenters the display.

Best of all, you can use each—the sump and the fuge—exclusively for their respective intended applications. In other words, you won’t ever have to dig through a 3-inch tangled mat of chaeto just to locate your heater. Or pluck bits of macroalgae out of the intakes of the filter components on an hourly basis. Combo models that incorporate both systems have become popular. Whichever way you go about it, installing a sump AND a refugium is a win-win. Squared. Ultimately, you’ll have a more capable and more accessible aquarium filter/plumbing system!