Advanced Beginnings: The Basics Of Water Movement In The Reef Aquarium

by | Apr 15, 2003 | 1 comment

If someone asked me what made a reef aquarium different from a fish aquarium one of the first things that come to mind, other than intense lighting, is water movement. Many of the invertebrates that we maintain in the reef aquarium are dependant on relatively strong water currents. In this article I will discuss some basic issues to consider when providing circulation in the reef aquarium.

Water movement is very important in the reef aquarium for several reasons. One of the most basic is gas exchange. The importance of oxygen import and the export of carbon dioxide cannot be over-stated. The accumulation of CO2 will depress the pH and can slowly eat away at alkalinity. Another important aspect of water movement is the removal of waste products from the corals and other sessile animals. Excess mucus from the corals as well as accumulated detritus must be swept away or they can suffer from a variety of problems including infection and/or suffocation. There have been studies that show that if the layer of water that surrounds these animals is not kept moving their rates of photosynthesis and calcification are deleteriously affected.

In addition to carrying away waste, water movement is necessary to carry food to the animals as well. In nature the animals would have plankton and phytoplankton brought to them by the currents. In our aquariums, the artificial currents serve the same purpose. They carry not only the food and supplements that we add to the aquarium, but the naturally accumulated detritus and plankton as well. These organic substances are a beneficial food source for many filter feeders. If however these substances are allowed to accumulate due to poor water movement, then they can encourage the growth of pathogenic bacteria. Now that we have established that good water movement is important, the next step is to discover effective ways to provide this movement.

In nature there are different types of water movement and different animals have evolved to prosper in these environments. Therefore, in order to maximize your success with these animals, it is important to find out what area of the reef they evolved in. This will give you a starting point for determining the type and intensity of water movement you will need to provide.

When most people think of movement in the ocean they think of waves. Waves are created by the winds and produce a very important type of water flow known as surge. This is the periodic rush of water that is produced when the mass of water in the wave passes over the reef. This back and forth type of movement can be very beneficial to the corals that we are trying to keep.

Another type of water flow is turbulence. This is the random swirling of water in all directions, which occurs when currents collide with each other or solid objects.

The most common type of water movement in the reef aquarium is laminar flow. This simply means that the water is moving in one direction.

In nature, this type of flow is generally found at deeper areas of the reef (>50ft). Most of the corals that are kept in reef aquariums are found in shallower areas (<40ft). Therefore, the majority of these corals will prefer surge and turbulent currents.


External pumps, such as these Turbelle pumps by tunze, work well for larger aquaria. They circulate large amounts of water with less heat gain than comparable internal pumps.

Today aquarist utilizes a myriad of devises and techniques to create these very important currents in the reef aquarium. In order for you to create an effective system it is important to understand the difference between filtration and circulation.

Filtration is the deliberate removal of particulates and dissolved materials from your aquarium water. For a more detailed description see the February 2003 issue of Advanced Aquarist. Circulation is simply moving the water around the aquarium. The reason that I make this important distinction is to draw attention to a common misunderstanding regarding the source of the aquarium’s water movement.

As with any complex subject in this hobby, people are always looking for (and giving) rules of thumb. With regard to water movement in reef aquariums, many authors will advocate at least 10 times the volume of the aquarium per hour. So if you have a 100-gallon reef, you should be moving around 1000 gallons/hour. This is a fine starting point as long as you understand that this does not mean that you need to move 1000 gallons of water in and out of your aquarium.

Many reef aquarists use some sort of overflow in the tank to take water to an external container called a sump. In the sump various tools such as protein skimmers, carbon containers, reactors, etc. filter the water. The water is then returned to the aquarium. This is referred to as an open loop. The volume of water that flows through this loop need only be 3-4 times the volume of the tank (not 10+). This is the filtration flow rate. The rule of thumb that was mentioned earlier refers to the circulation rate in the aquarium. This number takes into account, not only the return from the filter, but circulation from various other pumps as well. I cannot tell you how many times people have come to me and asked how they can quiet there filter down on their 90 gallon tank because they are trying to put 900 gallons/hr through their overflow.


This aquarium utilizes a closed loop circulation system. The intake is through the strainer located on the bottom of the photo and the return is through the bulkhead on the right hand side. In order to protect the pump, it is important to keep any intake strainers clear of algae and other debris.

It is important to remember that any sort of turnover formula that you use should only be a guideline. Match your circulation scheme to the types of animals that you will be keeping. For instance, many books will give information about where the corals are found in nature so that you can house them accordingly. The rating systems may vary from book to book, but you can get a general idea for a starting point. Once your tank is up and running, it will be important to experiment with the placement of your corals until you find an area where they prosper.

Let’s look at a hypothetical set up, and go over some important plumbing tips.

For example if you are setting up a standard 75-gallon reef tank you will need to give some thought as to the filtration/circulation components that you would want to include. Most reef aquarists find it helpful to install an overflow, which skims the surface water and takes it down to the sump. The flow of water into this sump is going to be equal to the amount of water that is added back into the aquarium.

The next logical question is “How many gallons per hour do I need to run through my sump?” This number is going to be dependent upon whatever you are using your sump for. A typical “Berlin system” that just uses a protein skimmer will only need the volume of water that the skimmer can handle. So if you have a skimmer that has 200-300 gallons per hour coming out of it, then running several times this amount past it is unnecessary.

If you are using your sump as a refusium, that houses a live sand bed or macro-algae, then theoretically you would only need to run enough water through it to maintain the same chemistry as the display aquarium. In other words, as long as you don’t see any differences between the chemistry of your sump to that of your aquarium then you are putting enough water through that filtration part of your system.

Keep in mind that most of your filtration is taking place in the aquarium, so that is where you should spend your dollars creating the water flow. Through the use of power heads or a closed loop system you will be able to quietly and efficiently create a great deal of water movement. As a general rule, it is better to produce this current from multiple outlets rather than one strong return.

Now let’s consider some various ways to move water around in the aquarium.

The first method that I would like to mention is the use of air pumps and blowers. The use of airlifts is a very cost effective way to move large volumes of water. These are two very important considerations for everyone. Unfortunately this type of system is not practical for most hobbyists’ displays. It would take a large number of airlifts to create the volume of water flow that we would like to have for our reef aquariums. And this volume of air bubbles creates a lot of salt spray that would create a very real maintenance issue. In my opinion however, airlifts are practical for commercial applications.

Most aquarists use multiple water pumps to create the necessary water movement in their aquariums. There are numerous companies producing just about any size pump that you would need. For most reef keepers this means a pump that can withstand saltwater, is quiet and inexpensive to run. Keep in mind that these pumps will be running 24/7 and on a larger system, the energy costs add up. So if you are designing a larger system, it will definitely pay to do some research. To learn more about pump terminology and design check out sights such as

The majority of reef aquarists will use either external pumps (outside the aquarium) or internal pumps, know as powerheads. These pumps come in all sizes and flow rates.

Power heads: There are a few brands, such as tunze, that produce air cooled pumps that mount above the aquarium. Although these are very powerful and do not heat the water as much as submersed types, they are quite expensive. When attempting to ascertain how many pumps you will need and how large they should be it is prudent to first look at the types of corals that you would like to keep, and from what area of the reef they are commonly found.

Scientists have used flow meters to find out what sorts of flow rates are common on various areas of natural reefs. These flow rates are given in inches per second which is a measurement of velocity. These velocities do not mean much to the general hobbyist since most of the pumps that we use are rated in gallons per hour.

Dana Riddle published an interesting article on water motion in the reef aquarium and arrived at some guidelines that I have found match my experiences. He suggests that aquariums with soft and large-polyped stoney corals should use (1) powerhead equivalent to that of a Maxi-jet 500 (rated at 1.75ft. /sec) per 24” of tank length. For aquariums that contain SPS corals use (1) powerhead equivalent to a Maxi-jet 1200 (rated at 2.45 ft./sec) per 12” of tank length. There are a number of submersible pumps on the market today and, like so many other aquarium products, I recommend consulting fellow hobbyists and your local fish store for advice.

Regardless of the type of pump that you choose there are a few things to remember. Always have some sort of protection securely mounted on the intake of the pump. A prefilter screen or sponge will prevent animals and debris from being sucked up to (or in to) your pump! Just be sure to make this prefilter easy to access for regular cleaning.

It is important that the pumps you select are water cooled (not oil). This way you never have to concern yourself with a potential contaminate. Also, since these pumps are water cooled, they will produce some heat. At this time I prefer the Maxi-jet line by Aquarium Systems for tanks less than 40 gallons. They are small, powerful, and relatively inexpensive.

If you are keeping a larger aquarium (over 40 gallons) then you may want to consider using a closed loop system. Basically this is when water is taken out of the aquarium and fed directly into an external pump and then returned to the tank. The return to the aquarium can be through a hole that is drilled into the side of the aquarium or simply over the back with a plastic pipe. Technically, a canister filter is a closed loop, but for these discussions I will be referring to circulation pumps only. Some advantages to this method are:

  • You can move a great deal of water without a lot of noise
  • There is less heat gain to the aquarium
  • This system is less unsightly since you don’t have pumps in the aquarium
  • You don’t have to worry about the suction cups that hold the powerheads to the glass wearing out and potentially damaging a coral head with a rough jet of water.

If you decide to use this type of system there are a few things to keep in mind:

  • Be sure to plan out the overall layout of the reef so that you can place the intakes and returns appropriately
  • You can run this type of system by simply running the intake and return over the back of the aquarium instead of drilling holes in the tank. This method is not as clean looking, but it allows you to set up this type of circulation on an existing tank.
  • If you do decide to drill the aquarium, do not drill these holes on the bottom of the tank, because if a pipe or pump housing were to crack you would loose ALL of your water (and hence your livestock). The closer you are to the surface, the safer you will be.
  • Be sure to include shut off valves and unions so pump maintenance or replacement can be done with a minimum of hassle.
  • Remember you can use your overflow boxes to hide the returns
  • Be sure that the pump that you are using is below the water level so that the front of the pump (refered to as the volute) will always have a supply of water.

In an attempt to create a more natural type of water flow (i.e. Less laminar and more surge-like) aquarists have developed a variety of methods and devices to randomize the currents produced by aquarium pumps. One of these is the use of a “wave maker”. This device gives power to the circulation pumps at random intervals thereby creating more random water patterns. This reduces the number of areas without good water movement (AKA dead spots). Although these devices do give the corals a break from one unidirectional flow, they don’t really turn on and off fast enough to really produce a surge action.


This new powerhead by tunze uses a propeller to create a current that has a broader stream than other impeller driven pumps. This design allows these pumps to move a large volume of water without the potentially damaging affects of a narrow discharge. These units also have an accessory controller that allows you to vary the speed of the motor, thereby providing a more surge-like environment.

There are a few potential drawbacks to these devices. Some pumps do not fair well with being turned on and off continually and will fail prematurely. Also, some powerheads have a noisy rattle during start-up and the use of a wave maker may drive you out of your living room. Be sure to seek out specific product testimonials from fellow aquarists or your local aquarium shop before putting one of these systems together.

In an attempt to create even more natural water movement in the aquarium, some aquarists utilize (aptly named) surge devices. These systems employ a container (usually a plastic trash can) that is positioned above the aquarium. Water from the aquarium is pumped into the container from a pump located in the aquarium or connected reservoir. When the container is almost full, a large PVC pipe in the container rapidly siphons the water back into the aquarium creating a rapid surge of water. This rapid flushing is a very effective way to reproduce the type of surge action found on reef flats. Although corals seem to do very well with this type of water movement, it does have some drawbacks.

Because the whole apparatus is above the aquarium it can be unsightly to say the least. Noise can also be a factor. The continual sound of flushing of water into the aquarium may be more than some can deal with. Then there is the matter of bubbles that create salt spray that gets on everything. This type of system would work well with an aquarium set into a wall, with an access room behind it. There have been a lot of modifications to these devices that make them more users friendly. To find out more about this type of set up you can check out for detailed plans.

Another creative solution to the issue of randomized water movement is the use of some sort of mechanized return system. One of my current favorites is the Sea-Swirl. This device attaches to the back of the aquarium and oscillates the return from your main return pump or closed loop pump back and forth once every minute. Although these units are somewhat expensive, they do solve many of the problems that wave makers have.


Basic Tips for Water Movement

Here are some basic tips to consider when plumbing your reef aquarium. I realize that this list could go on for days, and everyone has different opinions, but I am compelled to pass on a few tricks that might keep money in your pocket and water off your floor. They are in no particular order of importance:

  • Although it may seem like a lot of trouble at first, always take the time to properly size you pumps and piping. Although aquatic systems may operate with the improperly sized equipment, it is important to remember that all of this comes down to getting what you want and need for your animals and saving money. There is enough expense in this hobby without paying for oversized pumps and inefficient filters.
  • Try to avoid designing a system that needs constant attention. It’s always a good idea to make your sump large enough to go several days without a top off.
  • When installing filtration pumps, always use shut off valves and unions so that you can make pump service and replacement easy.
  • Be sure to cover any clear piping or reactors that are exposed to light with some sort of shading material to prevent algal growth, especially coralline algae.
  • Try to design a system that does not depend on any sort of check valves to prevent flooding. The high levels of organic and inorganic material that will eventually accumulate in your reef will tend to make these devices jam up. So avoid drilling the aquarium in any way that will make you dependant on a device that may fail. If you do choose to install a check valve for any reason, be sure to purchase one with a clear housing so that you can see whether it’s operating correctly. Also, just as with pumps, use unions and shut off valves so that you can service them easily.
  • Never undersize the pipe or fittings that are on the suction side of a pump. This is known as “starving” the pump. As a general rule, use whatever size fittings and lines match the intake with that particular pump. If there is a question consult the manufacturer.
  • Avoid making any pumps or prefilters hard to get to. This is especially important when trying to hide circulation powerheads in the reef itself. These pumps will need service, and whatever you stack over the pump will have to be removed in order to get to them. This can be especially heart breaking if corals, sponges etc. have grown in these areas, and have to be destroyed.
  • Always use some sort of ground fault interrupt. Salt water with electricity…enough said.
  • When plumbing the aquarium, avoid adding unnecessary bends and turns when ever possible. This adds unnecessary head pressure to the system and will cost more to run.
  • Always have a redundant source of circulation. Many people will try to circulate the water in their tank with only one pump. This may work fine if you have a large pump with several returns. Just remember that any pump can fail, and you never want to design a system that will leave your tank without any circulation if just one pump fails.
  • If at all possible, place drain and return lines either in an overflow box, or high up on the tank, so that in the event of a broken or leaking line, fitting or pump your tank won’t drain completely.
  • If you have to use hose clamps underwater, always use all plastic ones. The metal hose clamps (even stainless steel clamps) will rust.
  • It is important to periodically shut off the power to your tank to simulate a power outage. This will allow you to make sure that any back flow precautions that you have taken are still effective. This is important for many people that use small holes in their return lines to prevent back siphoning. These holes often become clogged and need to be checked on a regular basis.

I have to admit that working out the means of filtration and circulation for reef tanks has to be one of my favorite aspects of building a reef system. I’m sure that many of you will find that developing a system that works for you will be both enlightening and gratifying. I hope that by reading this article that you will take away some bits of knowledge that will help you to develop a circulation system that will make your reef aquarium a success.