What you’ll need when adding sump to existing tank.

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The popularity of the live coral “reef tank” aquarium has exploded in popularity over the past decade. Advances in aquarium keeping technology has made this notoriously difficult division of the aquarium-keeping hobby affordable and accessible to an increasingly large number of people. As the various devices and components become more affordable, they’ve also been resized to fit on some of the smaller and nano sized aquariums present in many people’s homes.

As a maintenance professional, one of the common issues that I have to address is under-filtered aquariums. I often see small aquariums with a live coral setup, with a filtration system that lacks a protein skimmer. Because this is not an ideal situation, my recommendation is always to add a skimmer to the display. While hang-on-the-back skimmer options are available, my preference is to add an “overflow box” and “sump” to the aquarium. The sump is a tank located underneath the main display aquarium, the overflow box is the device that safely (without floods) delivers the water to the sump tank below. In most instances, adding sump to existing tank is as easy as a few simple steps.

This setup allows one to add a larger and more effective protein skimmer than a hang-on-the-back skimmer will offer and increase the total volume of the aquarium at the same time. Both of which are huge benefits to the long-term success of a reef tank. A below the tank sump will also provide space for adding additional filtration devices further down the road.

This article discusses the basic steps necessary to add an overflow box and a sump to an existing aquarium. I’ll go over what equipment you’ll need as well as some links to my favorite prominent online retailers where you can find whatever you’ll need for your project. The purpose of this article is to help you determine what equipment you will need and allow you to develop a rough budget for your project.

Step 1

The first thing that you’ll need is what’s called an overflow box.


The overflow box creates an unbreakable siphon delivering water to your sump from the aquarium above. It will stop when you turn the pumps off (or during a power outage), and start right back up again when the pumps are turned back on.

Some aquariums have an overflow box built-in, called a “reef ready” aquariums. You can see in the following figure two different aquarium schematics.
Fish Geeks_adding sump to existing tank

The image on the right of the figure is an overview of a reef ready aquarium. The junction A is a special fitting called a bulkhead that allows you to plumb through a hole in the bottom of the aquarium. The water is returned to the aquarium through the blue line at C. This blue line is called your ‘return‘. and the water ‘over-flows’ through the teeth in the overflow box.

The image on the left of the figure is of an add-on overflow box, just like you will need to add a sump to an existing aquarium. Here, pipe at the bottom of the overflow box delivers water down to a sump below the aquarium. The water returns to the system through the blue return line at C, and it overflows into the overflow box just as outlined above.

The overflow box is a great economical way to add a sump to an existing aquarium and increase the volume of your system dramatically.

Step 2.

The next thing you need is a sump to catch all of the water that the overflow is delivering.


You could literally use any tank. If your tank is small enough (~40 gallons or less) a standard 10 gallon tank should work for you just fine. It is small, cheap, and leak-free.

For reef tanks, most people go “Berlin method” and use a bare sump. Others choose to use the sump to hold live rock, leaving more negative space in the display tank. Regardless, adding a sump will increase the volume of the system and provide you a place to put additional filtration.

Alternatively, sumps are available with features that can be very beneficial, including baffled chambers or sock filter hangers. The following is a link to some sump options for you to peruse. Sumps are extremely useful aquarium devices, and can be broken down into three basic categories:

Bare sumps are just that, empty. While some have integrated mechanical sock filters, these minimalistic options allow for the maximum accessible space for adding filtration devices. Generally glass aquariums are repurposed for simple bare sump setups.

By including a few baffles a refugium is created. Simply put, a refugium is another reservoir that you can fill with aquarium life. Some opt to fill their refugium with live rock, others cultivate algae as a means of nutrient export.

Finally, the bio filter integrated sump or “wet n’ dry” or “trickle” filter as they are affectionately known. For the majority of reef setups, Fish Geeks would NOT recommend using a wet n’ dry trickle filter. The reasons are a little complicated, and at the the very least beyond the scope of this article. The only caveat I will add, is that if you are running a small reef without a protein skimmer, adding a trickle filter will do you no serious harm. However, if you have a protein skimmer and a well balanced reef system, a wet n’ dry trickle filter is actually a little counter-productive. If you happen to pick up one of these types of sumps second hand, simply remove the bio-material (bio-balls generally) and operate the sump bare.

When adding sump to existing tank the size of the aquarium cabinet may preclude some ready-made sumps in some instances. Custom made sumps are an option in those cases.

Step 3.

Return pump


You’ll need a pump to feed water through your protein skimmer, and a pump to return water from the sump to the main display tank. A single pump can certainly accomplish this task, with the help of a few plumbing fittings.

submersible pump_adding sump to existing tank

Depending on the type of sump you have and your skills as a plumber, two options exist for your return pump. A submersible pump like the one shown above is the easiest option. These types of pumps generally come boxed with fittings for some common sizes vinyl tubing (tubing is available at your home center).

Pro Tip: If going this route, if the box doesn’t list the appropriate size tubing, bring along the male tube/hose adaptor fitting along with you when you purchase the tubing. This will ensure a tight fit, and eliminate the dreaded return trip to the store. (Don’t forget the pipe clamps or zip ties!)

The second pump option is the external pump. External pumps are most suitable to large and/or more complex aquariums. The main advantage of the external pump is that they dissipate heat into the air instead of into your reef tank. This is a key advantage for those who struggle with high aquarium temperatures.

Very generically, you will need a pump that has a flow-rate of at about twice the volume of your system in GPH (gallons per hour). Without taking into account any specifics of your system, this will give you between 1.5-2 turn-overs per hour. Where all the water in the system flows through the system up to two times per hour. (In reality it will be much less than this, but for sake of simplicity, I digress.)

Step 4.

The plumbing


Again, if you are using a submersible pump I’d recommend a simple vinyl tubing exchange between the sump and the display. The size of tubing you’ll need depends largely on the size of the pump you choose. Your submersible pump should come with a tubing adaptor. Go ahead and use appropriately size vinyl or silicone tubing for this fitting. It is a good idea to add a ball valve in the middle of this stretch of pipe to adjust the flow-rate of the water filling the aquarium.

For the actual interface with the tank I’d recommend using a nice commercial return assembly.

Pro-Tip: Drill a small hole (1/8inch) in your return assembly at a point inside the aquarium about an 3/4 inch below the water level. This will act as a “siphon-break” and prevent water from siphoning out of the system through the return assembly and causing a flood in the event of a power outage. Again, a commercial return assembly can be employed to dress up the return. My preference is to use Loc-Line components on all my return assemblies.

If you are using an external pump, your best option is to use PVC plumbing to connect the pump to your tank. Far less complicated than it sounds, a simple run to the tank is all that is required. Fish Geeks recommends using a check valve to prevent flooding in the case of failure.

You must also plumb a link between your overflow and the sump or the ‘return’ line we talked about at the beginning of the article. A stretch of flexible vinyl tubing is usually all that is required. Spa-flex hosing is a great option, as is the flexible PVC tubings which can be integrated into a standard PVC plumbing fitting.

Avoid any kinks or sharp turns. Use a diameter of pipe at least 50% larger than the diameter of the tubing on your return. Example, if your return is 3/4″ tubing, your drain should be 1″ or larger. If your return is 1″, your drain should be 1.5″ and so on.

Before you start your system, the siphon in the overflow box must be primed. The following is a link to a brief video that outlines how you prime a typical overflow box.


Step 5.

Start the pump.


Fill the sump with mixed seawater and start the pump. Water should flow through your return assembly into the tank at a brisk but not overwhelming flow-rate. Adjust as the flow as necessary.

If the aquarium looks as if it will begin to over flow, unplug your pump. Close the ball valve you installed slightly, and switch the pump on again. Wait for the system to “equalize”…you should notice the water level drop in your sump as the display aquarium is filled to the top. Top the system off with new water to maintain an appropriate “minimum-level” in your sump, or a few inches above your pump intake.

Its always a good idea to observe the system regularly for the first few hours to ensure the water levels stay equalized.

Adding sump to existing tank is just that simple.


After installing your sump, you are free to add your protein skimmer and other filtration devices to your sump.


Step 6.

Add a highly efficient protein skimmer.


There are lots of options out there. At a minimum, you can look for a “in-sump recirculating skimmer” as your best option here. New “cone shaped” varieties (as pictured below) offer a high capacity in minimal space. The centerpiece of your filtration system, it is important not to skimp in this area. Don’t be afraid to but a skimmer that has a capacity that is greater than that of your aquarium.
cone skimmer_adding sump to existing tank

Step 7.

Add a media reactor.


Another recommendation I make to all of my customers is to add a media reactor to your system. This will dramatically reduce nutrient levels in your system. I generally recommend granular ferric oxide (GFO) media that removes phosphates from your system. A new product, bio-pellets are a great way to reduce both nitrates and phosphates. Though the effect may not be as dramatic as with a GFO reactor.

With a sump, adding a reactor is very easy. Just add your media reactor filled with GFO media to your sump chamber.

You can feed the reactor with your main return pump if it is large enough to share capacity. Otherwise, a small dedicated submersible pump to feed the reactor is simpler way to go. Just connect the pump to the tubing provided with the media reactor and submerge it in your sump. The effluent water (the tubing coming out of the reactor) can be simply directed back into the sump.

The reactor and skimmer may float a bit before they’re filled with water. But once full, the weight of the water should hold them in place.

And that’s it. Well, at least that’s the basic version.

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