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Understanding Sumps
By S.K.Austin

Welcome everyone, to my first article for the community. Before I kick things off, I wanted to introduce myself. For those of you who don’t know me, my Name is Stephen, and I am a fishaholic. This is my first FFF meeting, and my hands have been dry for 0 days. :D All kidding aside, I am a 28 year veteran of the fish keeping hobby with the last 2 ½ years spent in reef aquaria. Unlike many who decide to try their hands at reef-keeping, I took things very slowly. I spent nearly 14 months researching as much as I possibly could in efforts to ensure my success. While I don’t claim to have learned, or to know, everything, I do feel that it was time very well spent.

As with other aspects of the hobby, I don’t claim to know everything about sumps. I do however, feel that I have learned and collected enough information to give even the most novice of hobbyists a fairly clear understanding of sumps and their applications in marine reef aquaria. With that in mind, this article is not meant to be the “be all and end all” collection of information on sumps, but rather a synopsis of some of the key issues concerning the use of a sump with your reef tank. If anyone is seeking a more detailed collection of information, I invite you to check out Reefkeeping Online Magazine, or follow the links highlighted in the references of this article. Now, on to the good stuff!

What is a Sump? defines a sump as:
a chamber at the bottom of a machine, pump, circulation system, etc., into which a fluid drains before recirculation or in which wastes gather before disposal.

For the purposes of the fish and reef keeping hobby, a sump is simply a second tank, tub or other storage container, housed below the main display tank. The water from the display is continuously drained into the sump by various means and for numerous reasons, all of which we will discuss later. Once in the sump, the water is processed for whatever purposes the sump was intended, and is then pumped from the sump, back into the display.

In most cases, a sump is a glass or acrylic tank, divided into two or more chambers, and housing a Skimmer, Heater, and Return Pump. The walls between each chamber are called “Baffles”. Below are a few photos of a simple sump designed from a 20 gallon fish tank, and a slightly more elaborate design from a 30 gallon fish tank.

20 gallon sump with refugium before, and during operation.

30 gallon sump with refugium before, and during operation.

What is the purpose of a Sump?
There are a number of reasons people may incorporate a sump into their reef system. Sumps can serve different purposes depending on their design, but there are several universal benefits that all sumps offer regardless of their design or type. In general, a sump’s purpose is to aid in filtration, increase the total water volume of the system, and provide a place for equipment outside of the display aquarium.

What are the benefits of operating a Sump?
As stated previously, there are a number of benefits that are, or can be, attained with the use of a sump. Some of these benefits are universal while others are design specific.

• Increase in total water volume
• A place to hide equipment outside of the main display
• Constant water level in the display
• Overflow skims protein buildup from the surface
• Increased aeration and circulation of the water
• Safe place to add your chemicals and supplements
• Temperature reduction
• Refugium and propagation chambers
• Mechanical filtration

First and foremost atop the list of benefits is the increase in total water volume. Any experienced reef keeper will tell you, the smaller the system, the quicker problems progress. A larger water volume provides a much more stable environment, allowing for slower changes in water chemistry. One gallon evaporated from an 85 gallon system (55g tank + 30g sump) will have much less impact on the water’s salinity than in a 55gallon tank alone. In another simple example, a single drop of ammonia will yield a far higher concentration in a 55 gallon tank alone, than that very same drop in a 55 gallon tank with a 30 gallon sump incorporated into it.

A close second on the niceties gained by the use of a sump is that they provide a place to hide equipment outside of the main display. Let’s face it, that thermometer probe, heater, and skimmer pump do little to improve the aesthetics of your display. In fact, they make your system look downright trashy. It’s almost like buying a brand new Alfa Romeo, and donning it with a Domino’s Pizza Delivery roof light. We spend thousands of dollars creating and maintaining a beautiful reef system. Let’s make sure techy gadgets have a place to hide. Calcium reactors, UV sterilizer pumps, Kalkwasser stirrers can all be incorporated into, on, or around the sump.

Yet another inevitable benefit is the constant water level in the display. Water within the display exits the system by traveling over a stationary overflow. Since the water is consistently being replenished in the system from the return section of the sump, any evaporation that occurs in the system will only be observed in the return section of the sump. No more sinking water levels and unsightly hard water buildup around the top of the display. The Overflow also provides a valuable service in that it skims proteins from the surface. A clean surface is beneficial for several reasons. The obvious reason is aesthetics. Nobody likes to see scum buildup floating atop their aquarium, but that scum blocks out valuable light and impedes on the gas exchange and negatively impacts the oxygenation of the system.

You can also expect increased aeration and circulation of the water in the system. That long trip from the overflow to the sump does a very effective job of churning the water and air together. Bubble traps between sections in the sump then help to eliminate the micro-bubbles from returning to the display. The return lines also provide flow within the display alleviating the need for additional powerheads.

A sump also gives you a safe place to add your chemicals and supplements. When chemicals are added directly to your tank, they can come in contact with your livestock at levels that could be detrimental to the health of the animals. By adding your chemicals via the sump, you are allowing the chemicals to be mixed thoroughly into the water column before they reach the display. Steps like this can prove beneficial to the long term health of your tanks occupants.

In addition to the inevitable benefits, there are a few other benefits that you may seek to incorporate into your sump. While these features are not always attainable with every design, you should definitely keep a few things in mind when designing, or choosing a design for your sump.

Depending on the space available for your sump, you may wish to choose a design that incorporates a Refugium Section into it. This is a section most often used for the growth of macro algae for the purposes of added nutrient export. Refugiums are also beneficial to the proliferation of Amphipods, Copepods, Isopods, Mysis, and other “reef bugs” which act as a natural food source for many fishes. In some cases, a refugium can also act as a safe haven for young, weak or injured animals and newly propagated corals.

Another idea, depending on space, design, and the hobbyists’ specific needs, is the incorporation of a Propagation Chamber. This is an area, much like the refugium, solely intended for storage and safekeeping of newly propagated corals. As you begin your venture into reef keeping, you will likely see little need for such a feature. But in due time, you will find yourself needing to cut corals back to prevent crowding, or shading. Learning to properly propagate and store these coral “frags” will give you a resource with which to sell or trade, and perhaps help reduce the burden the hobby may have on your pockets.

The need to add fresh top-off water to replenish water lost to evaporation is never ending. The need to maintain a stable salinity requires that top-off must be done every day, if not twice a day. With the use of Auto top-off systems, the task of replacing water can be reduced to refilling a spare 10 gallon tank once or twice a week.

The last benefit I’m going to cover here is mechanical filtration. There is a lot of debate as to just how beneficial any mechanical filtration, that’s been incorporated into the design of a sump, really is. Filter pads, Bio Balls and filter socks are often discouraged as they tend to aid in the build-up of nitrates, especially when not maintained properly. I use a filter sock for water polishing, and I change it out every 3 days religiously. I think the results are worth the efforts, but that’s my opinion.

As a final aside, many reef hobbyists who employ a sump have reported an average 2 degree reduction in the overall temperature of their system. While some of the time, this may prove to be no great benefit, this can be very helpful for those of us aquarists who are unable to operate a Chiller during the hot summer months. The added surface area provides for more cooling via evaporation. Additionally, frozen top off water or bottled ice can be added to the return section for further cooling.

What about the negatives?
There are none. Well, ok, that’s not entirely true. But I will say that I have yet to hear of any negative aspect to having a sump that would cause me to consider going without. When properly installed, the benefits of a sump far outweigh any risk or downside. That having been said, I’ll share the few negative points that I’ve learned of along the way.

Added noise is usually a feature that is accompanied with the use of an overflow and sump. All that water cascading into overflows, churning its way through the plumbing, and gargling into the sump can create the sensation of sitting alongside a babbling brook in the woods. I don’t care how relaxing it sounds, it’s not! As a matter of fact, it’s downright annoying. The good news is, with a bit of investigation and research, most of the noise can be taken care of and regardless of what some may say, a reasonably quiet system can be attained.

As long as your sump has been properly designed and installed, the chances that you will experience a flooding situation are minimal. Be that as it may, it is still a risk involved with operating a sump. Flooding can happen for a number of reasons, but all of them are quite preventable. The bottom line is, if you experience a flood, chances are it’s your own fault. I’ll cover “preventing a flood” later in this article in the section titled “Designing and Building Your Own Sump”

Another negative point to having a sump is that, in most cases, it completely eliminates the storage space below your tank. Storage is a valued commodity, especially in the hobby of fish keeping. This holds exceptionally true for us reef aquarists. Foods, nets, propagation tools, and the abundance of chemical additives all need a place to be stored. The tank stand makes a convenient space for all of those items you use on a daily basis.

Salt creep is another downside. Salt creep occurs when saltwater, settling on surfaces above the water surface, evaporates. Since salt does not evaporate with the water, it is left behind as a crusty white icing. Salt creep is especially prevalent in and around the inlet (the end of the drain line) of the sump. This is because bubbles and microbubbles created by the turbulent trip through the drain line and skimmer outlets burst at the waters surface. Tiny droplets of water expelled from the bursting bubbles settle and evaporate on any and every exposed surface within range. I have found that by incorporating a cover for the inlet and skimmer sections into the design of my sump, salt creep outside of those sections can be all but eliminated.

The added cost and maintenance is yet another drawback to adding a sump to your system. Prefabricated sumps are not cheap. At least not any sump that your Local Fish Store (LFS) wants to sell you. That coupled with the need for a return pump, and the addition of an overflow may often put the possibility of a sump out of reach for some of us. Regardless of the final cost of your sump, it is still yet another piece of equipment that will need to be maintained. One good thing here, and something your LFS won’t often tell you, is that a good sump is relatively inexpensive to make yourself and it’s not all too difficult to do. That is, if you trust your DIY skills.

How does a sump operate?
The first thing to understand is that a sump is fed by gravity. This means that the sump needs to be lower than the display aquarium. The water from the aquarium exits the tank by means of an “overflow” (I’ll talk more about the overflows and how they work later in this article). The water travels down to the sump via the drain line where it is channeled through different chambers and processed in whatever manners employed by the design of the sump, and the equipment housed within it. Finally the water is pumped by a return pump, back to the display aquarium via the return line.

Notice, in the diagram above as well as in the earlier photos, that there is also a line with a valve from the return line back to the inlet section of the sump. This feature has 3 main purposes. By opening the valve, you decrease the water returning to the display. This allows you to adjust the flow in the event your pump is rated higher than your overflow can handle. It also allows you to isolate the sump from the display without disrupting flow through the sump (this requires additional valves on the drain line and return line for proper employment). Isolation from the display is especially important when harvesting or pruning macroalgaes from the refugium, or cleaning the sump. When the sandbed in the refugium is disturbed, gasses and protiens are introduced into the watrer column that, if allowed to reach the display, can be very harmful to your aquarium's inhabitants. I highly recommend that, when plumbing your sump, you consider this option in the design.

What is an overflow?
There are in general, three types of overflows available. The Prefilter, sometimes referred to as an “External Overflow”, employs a U shaped tube to create a siphon that carries the water over the side of the tank. The Internal Overflow, commonly seen in “reef ready” tanks, has a chamber with drilled holes (most often in the bottom) in which simple plumbing is needed to carry the water to the sump. The Coast to Coast Overflow is a newer style overflow similar to the Internal Overflow, but that travels horizontally across the upper back of the tank. The Coast to Coast overflow requires holes to be drilled in the back of the tank.

The Prefilter or External Overflow

The internal overflow works by siphon. Water from the display enters a box on the inside of the tank. The water is then drawn up over the side of the aquarium via siphon in an inverted U shaped tube to a box on the outside of the tank. The water then drains down to the sump. The drain pipe sits above the U-tube so that water never drains completely out of either of the two overflow boxes. This prevents air from entering the U-tube causing a siphon break. In the event of a siphon break, water draining to the sump will cease while the return pump continues to return water to the display until the return section of the sump is emptied. This usually results in a flood of the display aquarium and the eventual burning out of the pump (hopefully not resulting in a fire). As long as the overflow is designed properly, a siphon break should not happen. One disadvantage to the External overflow is that because the water is forced to travel up and over the tank, the flow rate will be less than that of the gravity fed “Reef Ready” internal overflow.

The question often arises “How do you prime (start) the siphon in an external overflow?” The answer is quite simple. You need only a section of airline tubing about twice as long as the length of the U-tube. Insert the airline tube into the inverted U-tube to the top of the bend, then place the U-tube into the overflow. With both sides of the overflow filled with water, suck the air out of the U-tube with the airline tubing. As you suck out the air, you will see the U-tube fill up with the water from the overflow boxes. Once the water crests the bend in the U-tube, the water will begin to flow. If you are not quick in sucking out the air in the U-tube, when the water begins to flow, the remaining air pockets will begin to move in the current making it difficult to remove the remainder of the air. Smaller air bubbles in the U-tube will eventually work their way out in the current, but larger bubbles have the potential to remain and collect air bubbles entering the tube, potentially leading to an eventual siphon break.

The Internal “Reef Ready” overflow

The “Reef Ready” tank is equipped with an internal overflow. Water flows into a separate chamber within the display. Holes drilled in the glass bottom of the tank within this chamber allow plumbing to go through the bottom of the tank rather than up and over the side of it. This allows for a greater flow rate to the sump. Since the water is traveling straight down through the tank, there is no need to prime the overflow, and no worry about the potential flood resulting from a siphon break. Generally, the overflow chamber is located in the corner, or against the back of the tank. Some larger display aquariums may have the overflow chamber positioned in the center of the tank, which is ideal for tank planned to be viewed from both sides. If you’re looking to start a reef tank, the reef ready tank really is the way to go. That is unless you have the resources to install a coast to coast overflow.

The “Coast to Coast” overflow is another type of internal overflow, where the chamber spans the entire upper back of the tank, and the holes are drilled in the back of the tank. The benefits to the coast to coast overflow over the standard reef ready internal overflow include increased surface skimming, reduced noise, improved aesthetics (no bulky black chamber), and it allows more room for the rockwork in the display.

Understanding the purpose of “Siphon Break”?
If your sump (and it's related plumbing) was designed properly, a power outage should not cause a flood. When you experience a power outage, the return pump will cease (obviously), as will your skimmer pump. The water in the tank will then drain to the level of the overflow. Usually, that’s about 1/2 inch below the normal operating level. So long as you have a siphon break (a hole in the return line just below the water level of normal operation) the tank should not drain beyond this point. A properly designed sump will have accounted for this occasion and should be able to house any water that it may accumulate from the tank, the skimmer, and any plumbing.

Problems generally occur when people either fail to install a siphon break, or their siphon break hole becomes plugged over time. When this happens, the return line will act as a siphon and the tank will continue to drain until the waterline reaches the return outlet. Once the water reaches that point, air will enter the return line, thus breaking the siphon. Unfortunately, in most cases the return outlets are several inches below the minimum level of the overflow, and our sumps cannot handle that much excess water, so we end up with a flood.

So the key to remember here is, Make sure you have a siphon break hole drilled in your return line, and make sure you periodically check, and clean it out.

What can I use for a sump?
People often ask what types of containers can be used for a sump. Too often I’ve read the response “Any container that can hold water can be used for a sump.” While this may seem acceptable, it is most certainly not true. Some people like to use water troughs for sumps. A water trough can make an excellent sump, but some manufacturers add an algaecide to the plastic from which the trough is made. This can cause problems for the health of your livestock. Used aquariums that have been treated with copper based medications are also a poor idea for a sump as copper absorbed into the silicone can leach out into the water and poison copper sensitive invertebrates. A far safer statement would be “Any food safe container that can hold water can be used for a sump.” Of course there are some considerations that must be made in choosing a proper sump.

What are the considerations for choosing a sump?
While it is true that nearly anything can be used as a sump, there are considerations that can make any number of the options not feasible for your specific application. The most obvious consideration is that the vessel to be used must be able to provide the functions you need for your system. If your sole purpose for adding a sump to your system is to hide a skimmer and a heater, and to add to your total water volume, then any food safe container large enough to house those items and your return pump will be sufficient. If however, you are looking to include a refugium, propagation chamber or other such niceties, you may need to consider more expensive options. Acrylic sumps can be costly, but can usually be found already designed to suit nearly any need you could possibly fathom. Designing your own acrylic sump will allow you to make the most of the space you have available, and can be designed to suit your specific needs. While designing and building an effective working sump out of acrylic will require a bit more work, it will save money as opposed to purchasing one (providing you do it properly from the start). My personal choice for a self-constructed or DIY Sump is the use of an empty aquarium with acrylic baffles held in place by aquarium silicone. This is relatively inexpensive, yet provides a sturdy, easy to construct design that can be tailored to suit your specific needs.

Available space is always a consideration that must be made. If you have no room for a 20 gallon tank under, behind or beside your tank stand, then you obviously cannot use a 20 gallon tank for a sump. Lack of space seems to be one of the leading reasons that reef aquarists choose to go without employing the use of a sump. This is especially true of the younger generations still at home with their parents and keeping their tanks in their bedrooms. Luckily, you don’t need a sump to be successful reef aquarists, and the lack of one teaches our younger hobbyists to be religious with their maintenance.

The flood factor is a consideration that is almost always overlooked by new aquarists. There must be enough room to allow for excess water to drain through the system in the event of a power outage. When the power goes out, the return pump stops and the display drains to the level of the overflow. Additionally, the skimmer pump will stop and the skimmer will drain back into the sump. The sump must be able to handle all of this excess water without flooding. If you are designing your own sump, it’s helpful to know the formula to calculate volume ( (Height x Width x Depth) / 231 ), and plan to leave enough spare room to handle the extra volume. If you already have your sump prepared, and are looking to ensure you can handle the power outage event, there is a simple trick you can do.

1. Get your system running with the minimum level of water needed for proper operation.
2. Cut the power and allow the system to drain into the sump.
3. Once the system stops draining, fill the sump to a comfortable maximum fill level (an inch or so below the rim of the sump).
4. Restore power to the system.
5. Once the system is back to normal operation, mark the level in the return section of the sump.
6. This should be your maximum fill level when adding top-off water

The evaporation factor is yet another consideration that is often overlooked, but must be made. The return section of the sump must be able to house enough water to keep the return pump fully submerged, plus enough to cover maximum evaporation loss for at least 24 hours. In other words, if it takes 1.5 gallons of water to cover your return pump, and you loose a maximum of 2 gallons of water per day to evaporation, then your return section needs to house at least 3.5 gallons of water without exceeding your maximum fill line. For the record, if you are using forced air heat, the coldest winter months should yield the largest loss to evaporation as indoor humidity will be very low at that time of year. If your return section cannot meet that requirement, there are steps you can take to reduce evaporation. Use of tank covers will help so long as you can keep water temperatures in check. Use of a humidifier in your home will also help to reduce evaporation.

Designing your own Sump (a few rules of thumb)
If you are planning to design your own sump, there are a few rules of thumb to keep in mind. Some of this may look a bit familiar from earlier sections. Some of this is completely new. All of this will be brief, as I plan to cover these details more in depth within my next article titled “Building Your Own Sump”. So the things you want to keep in mind if you are planning to design and build your own sump are as follows:

• Build the sump as large as you can possibly accommodate. The larger the better.
• Consider all of your needs before you start building. Moving a baffle sucks!
• There must be enough excess space to house any water that drains out of the display, the plumbing and the skimmer, in the event of a power outage.
• Inlet chamber need only be large enough to house the inlet pipes and the skimmer.
• Bubble trap baffles should be 1” apart (or more)
• Refugium section should house at least 10% of the display tank’s total volume. (55g tank = 5.5g Refugium)
• Return section must house enough water to cover 24 hours of evaporative loss without exposing the return pump.

I hope that you've found this article helpful. Please, if youve taken the time to read this article, take a few moments to provide a rating, and perhaps send me your comments. If you have any questions, comments or suggestions regarding this article, please send me a Private Message. I would be happy to elaborate on the details contained herein, or consider any suggestions on additional information than may not have been covered in this article. Thanks for reading, and Happy Reefing!


Reefkeeping Online Magazine – January 2008:
Reefkeeping 101 - Sumps! - By Marc Levenson

Reefkeeping Online Magazine – January 2003:
Everything You Always Wanted to Know About Sumps, Part I - By Gregory S. Taylor

Reefkeeping Online Magazine – April 2003:
Everything You Always Wanted to Know About Sumps, Part II - By Gregory S. Taylor

Reefkeeping Online Magazine – July 2003:
Everything You Always Wanted to Know About Sumps, Part III - By Gregory S. Taylor

Sumps – Dr_DBW

Melev’s Reef
How does a sump work??? - By Marc Levenson

American Psychological Association (APA):
Sump. (n.d.). Unabridged (v 1.1). Retrieved May 29, 2008, from website:
Chicago Manual Style (CMS):
Sump. Unabridged (v 1.1). Random House, Inc. (accessed: May 29, 2008).
Modern Language Association (MLA):
"Sump." Unabridged (v 1.1). Random House, Inc. 29 May. 2008. <>.
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