What is a Closed-Loop Aqaurium?

I remember having this same question many, many years ago about what was closed-loop reef tank? And if you are here, you probably have a bunch of questions like what they are too? What is their purpose? Are they worth it? And are you missing out on something by not using a closed-loop system?

A closed-loop reef tank is a saltwater aquarium that has been fitted with a piped-in water circulation system. Closed-loops create high-flow water movement necessary for fish & coral health while keeping the aquarium free of obtrusive pumps & equipment. They are best suited to large aquariums.

If you want to learn more about closed-loop systems, continue reading below, where I go into more detail about everything you need to know.

What Is Considered a Closed Loop Reef Tank?

Closed-loop systems are piped circuits in which the aquarium water is circulated throughout the system without it being exposed to the outside environment. With closed-loop reef tanks, all inlets and outlets are constantly submerged and comprise one continuous loop.

Rossco’s Closed-Loop System on Reef2Reef.com Aquarium Forums

The aquarium water is pulled through one or several intakes by an externally mounted water circulation pump and then is pumped back into the tank through one or several nozzles.

Usually, a significantly greater water flow can be generated with a closed-loop system compared to using individual powerheads and wavemakers, especially on long or large aquariums.

In comparison, the most common reef aquarium setup using a sump is considered an open-loop system, because the water that is pumped through the system is exposed to the outside atmosphere when it reaches the sump.

Aquarium Sump Operation
An Typical Open-Loop Tank & Sump Setup

That being said, closed-loop systems and open-loop systems shouldn’t be viewed as systems that cannot coexist. In fact, many aquarists use them together—the closed-loop system provides water flow, and the open-loop system the filtration.

An Open & Closed-Loop Reef Tank Setup

Why Are Closed Loop Reef Tanks Used?

Closed loop systems are used mainly for creating water flow within the main display aquarium.

To a certain degree, closed-loop systems can be considered the old-school way of providing water flow in larger tanks because back then, we didn’t have the powerful wavemaker pumps we have today.

Closed-loop systems are mainly used on large and usually custom-built reef tanks because the holes have to be pre-drilled before the glass gets tempered. Because of this most closed-loop systems will be found on the larger 100 to +800 gallon reef tanks.

Closed-loop systems are usually either located underneath or behind the tank. Placing the closed-loop pipework and circulation pump under the tank is preferred because it is more aesthetically pleasing, allows the tank to hug the rear wall a little more tightly, and generates good water flow that will not allow the detritus to settle down.

Closed-loop reef tanks are not as common as they used to be used because they require a lot more work and cost to install but, in return, allow for clutter-free and smoother running display tanks. It all comes down to the personal preference and budget of the aquarium owner at the time they install the tank and filtration.

Advantages and Disadvantages of Closed Reef Tanks Systems

Clutter-Free Display Tanks

Most people don’t really enjoy the sight of powerheads, especially me!. Even though some powerheads can be hidden within artificial rocks, or behind the aquascape, they are still not very pretty to look at. If you can hide the pump, you usually then have the cables to look at!

To me, a reef aquarium should display the beauty of the living world within it with as few mechanical devices on show as possible!

Since you do not need to use powerheads and wavemakers with a closed-loop system, this allows you to keep the clutter in your display tank to an absolute minimum.

Maintenance and Care

The routine maintenance and care on closed-loop systems, if properly set up, is very easy to do. Ball valves must be placed before and after the circulation pump(s), allowing for easier mounting, dismounting, and servicing.

Ball valves should also be turned weekly to prevent seizure over time! One owner I knew did not do this, then when his closed-loop pump died he had a heck of a time unseizing that valve!


Leaks are possible with any type of aquarium, but as long as the bulkheads and the piping are installed correctly, there shouldn’t be any leaks, this is the same no matter the type of system installed.

If you turn the power off on a closed-loop reef tank, the water level will not change—only the water flow will stop.

In comparison, in an open-loop reef tank, not only will the water flow stop, but the water level will also drop until it goes below the overflow. This means that a sump flood can happen if setup improperly because it is an open-loop system.

Water Flow

Depending on what corals and type of fish you have in your tank, you will need different amounts of water flow—which can easily be achieved with a closed-loop system. By using nozzles and valves you can direct and control the flow in many parts of the tank.

This means that you do need powerheads. If the closed-loop system is set up properly, you can generate more than enough water flow to keep an SPS and LPS reef vibrant and healthy, even in the largest aquariums!

However, alternating the flow can pose some difficulties compared to using powerheads, which you can place and direct in any direction you see fit. This is why owners often install flow diverters on closed-loop systems, like the ones from Oceans Motions and SCWD, to keep the water flow more natural and randomized.

How An Oceans Motion Works:

What The Water Flow Does:

You can find out more information and pricing on the Oceans Motions Flow Diverters Here at AquriumSpecialty.com

Noise Levels

Although most powerheads are fairly quiet, it is not uncommon for them to make strange noises, especially when they are ramping up and down in flow. Keeping that in mind, a closed-loop system should normally be quieter, with just a constant hum from the circulating pump.

If your aquarium is going in a noise sensitive area then you are best advised to join your local reefing club and visit tanks that run both closed loop systems and ones that just run powerheads to see for yourself.

There is no getting away from noise sometimes. Although the new wavemakers are incredibly quite, so to are the new circulation pumps.


The number of holes drilled can vary between two to ten or more in some instances, with the most common being between four to six holes—two drains and two to four returns. The drains are typically 1.5 inches in diameter, and the returns 1 inch. Drilling holes with a larger diameter is often avoided for safety reasons.

Drilling the tank is one of the things that should be considered carefully. Once you drill the tank, you cannot go back; it’s not reversible, and mistakes can be costly.

Drilling a tank is easy, but drilling should be done only on untempered glass because drilling tempered glass will shatter it, with explosive power! This is why most closed-loop systems are designed before the tank is manufactured as almost all bottom panes in an aquarium are tempered for strength.

It is possible to make a closed-loop without drilling but that usually takes away all the aesthetical advantages and appeal closed-loops can offer. Usually, the pipework is going to be a little more complicated to set up, and it will have to go up and over the top of the tank, so it is not a pretty sight to look at, but it does work. With careful cabinet construction, you can hide a good portion of the pipework within the canopy, only seeing the nozzles under the waterline.

There are many great DIY examples out there, but there are also some horrible ones too!

A Single Point of Failure

The quality and type of pump you will be using is extremely important! This is not the place to cheap out on!

You want to use a high-quality pump that can run 24/7 for years without failing because if it fails, it will prevent the whole system from working. Losing your only source of flow can be fatal for your coral.

Because of this, many closed-loop owners will have a spare identical pump all plumbed and ready to be swapped out at a moment’s notice.


It is arguable whether or not a closed-loop will be more cost-effective in terms of providing water flow compared to using powerheads.

A less powerful powerhead is still viable on smaller aquariums, but on larger tanks where many powerheads may be necessary, installing a closed-loop can potentially be cheaper. It’s a matter of calculating the wattage used.

The more powerheads you run, the more watts are consumed. Compare that to the single wattage of the closed-loop’s circulating pump and you could be saving quite a lot of money running just the one pump.

The other cost to consider is the initial setup. Having the tank drilled, pipework installed, a circulating pump, spare pump, and maybe a flow diverter could run you around $1000-1500, however, just a single powerhead/wavemaker can run you $300-700 and depending on how many you will require may make a closed-loop cheaper.

Is a Closed Loop Reef Tank Worth It?

In my opinion, closed-loops still have their place. They are neither obsolete nor a relic of the past and they are arguably one of the best ways to keep a clutter-free tank while providing optimal levels of water flow, especially if the aquarium is long or large. If properly set up and designed, you will not see any pipes or power cables!

But powerheads have improved in quality and performance over the years to the point where they—although not visually appealing—can be considered a good alternative since they are super easy to install and can provide tremendous amount of flow, especially the Gyres!

For More Information & a Great Selection Gyres, See them Here at Amazon.com

The reality is not many owners will be comfortable with or willing to drill multiple holes in the rear panels (bottom panels are tempered remember) of their tanks to set up a closed-loop system. As a result, closed-loop systems have come a little out of favor and are not so commonly used today. This makes closed-loop systems more of a hassle to initially set up, but they are still worth considering, nonetheless, especially if you love to DIY!

To Finish

Closed-Loop Systems are a great way to provide tremendous and sometimes alternating flow to a reef aquarium. By using a pump, a few drilled holes, and some plumbing you can really create a stunning aquarium with zero equipment in sight.

Primarily used on larger aquariums where the installation of multiple powerheads would be required, a closed-loop system can make aesthetic and financial sense. But the cost and design of a custom aquarium can then make a very unattractive option to most reef tank owners.

For small aquariums, powerheads and wavemakers are the best route. For long and large aquariums, the Closed-Loop is a great alternative.

Further Reading

If you found this article helpful, please take a read of some of my others:


Hi, I'm Richard and I have been an avid aquarist for over 30 years with a passion for Saltwater Aquariums. I love to pass on my knowledge to help others get the same amount a pleasure out of this hobby as I do. View my About Me page to find out more about me & my mixed reef aquarium.

Recent Posts