There is a level of scientific understanding that is gained as you mature in the reef aquarium hobby. For many, this real-world application of science is one of the most attractive aspects of a reef tank. Its starts small with things like the nitrogen cycle and understanding various water parameters and builds into much more complex concepts like maintaining water chemistry and understanding how to optimize your LED lights. 

Water flow is an area that is deceiving in its complexity and necessity for scientific understanding. While creating flow in your aquarium is not inherently difficult, making the most efficient use of your pumps and providing optimal flow conditions for all of the animals in your tank is much harder than it looks. It gets even more difficult as your corals grow and begin to change how the water moves throughout your tank.

Having a scientific foundation of water flow and why your aquatic companions need this flow makes things much easier when it comes to being a confident and capable reef aquarist.

The Benefits of Water Flow in Reef Aquariums

There is a clear set of scientifically proven benefits of water flow inside our aquariums. 

  • Increases rate of calcification among corals
  • Delivers energy via food and nutrients
  • Helps release and export waste
  • The primary method for gas exchange and keeping the water oxygenated
  • Prevents coral poisoning
  • Sustains vibrant coloration and optimal growth rates

Boundary Layer

1. The boundary layer

Defined by physics and fluid mechanics, a "boundary layer" is a layer of flowing fluid in contact with a surface. It's somewhat abstract to describe in words, but in the case of a coral in your tank, this layer surrounds the coral's surfaces and is created by the water coming in contact with the coral. The water flow itself changes (velocity and type) as it touches the coral's surface but it also experiences a shift in the concentration of various molecules because this is where the water interacts with substances being released by the coral.

This boundary layer is thicker in lower flow conditions and thinner under higher flow conditions; in terms of coral's biological success, the thinner layer allows for more optimal growing conditions. If the velocity of flow is insufficient this thicker boundary layer inhibits the rate of gas exchange, nutrient import, and waste export ultimately slowing the growth of the coral. Creating sufficient flow conditions in your tank is all about penetrating this boundary that exists around your corals. 

Effective flow pattern

2. Precise tank turnover rates are a pipe dream

Attempting to gauge sufficient water flow in an aquarium with a number is just not practical when it comes to internal flow. You may encounter recommendations such as "10x - 20x total tank turnover per hour" in reference to how much flow you need to achieve.  The reality is, these tank turnover metrics are useless because the rate of flow has no influence over whether or not the flow is actually coming in contact with the corals. Ultimately, how you use that flow is what really matters and the goal is to produce movement throughout the entire tank. 

For example, if you have 2000 GPH of flow pushing across the front side of your aquascape, this is not doing any benefit to the corals on top or behind your aquascape because they are not coming in contact with that flow. In the diagrams above, note how effective pump placement based on your aquascape is far more important than flow rate. This is so important in fact, you should design your aquascape around your planned pump placement and flow patterns.  

3. Sufficient flow helps corals export toxins

The process of photosynthesis by the zooxanthellae within coral naturally results in a build-up of toxic oxidants as a byproduct. A coral expels these oxidants through its surface tissue and without sufficient water flow, the rate at which corals can expel those oxidants through its surfaces is inhibited.

This is exactly why we see a correlation between high light and high flow. The more light a coral receives, the faster the rate of photosynthesis. Under these conditions, higher amounts of flow are required to sufficiently export the increased level of toxic oxidants. Should the production of oxidants exceed the rate at which a coral can get rid of them, intoxication occurs. 

4. Water flow prevents acidification within the coral

Surrounding the coral with flow allows the coral to eject excess Hydrogen ions preventing acidification within the coral. The process of calcification results in extra Hydrogen ions and if the coral is not able to get rid of these excess H+ ions, acidification occurs which ultimately slows down or halts the calcification altogether.  Long story short, insufficient flow slows down coral growth. 

5. Penetrating the boundary layer delivers elements to the corals

Corals absorb elements from the water through their surface tissue. These elements must be available in the boundary layer because this is what comes in contact with the coral's surfaces. Sufficient flow that penetrates the boundary layer will ensure the right concentration of elements. 

SPS coral

6. Organic nutrients are also delivered via the boundary layer

Similar to major, minor, and trace elements, nutrients such as amino acids are delivered to the coral through its tissue.  The boundary layer must contain these nutrients in order for the coral to absorb them.  

7. Corals grow faster under adequate flow

It goes without saying....corals that receive adequate flow grow the best. You can see this in your tank and sometimes even within the same coral where one side of the coral grows much faster than the other because of the exposure to flow. 

8. Each coral is unique in its flow requirements - more is not always better.

It's no secret that SPS corals prefer much higher flow compared to LPS and many soft corals. When it comes to creating optimal flow conditions, it's important to consider the type of coral and the flow conditions it prefers to achieve the best possible growth. More is not always better when it comes to flow, even among SPS. Breaking the boundary layer still applies, its just relative to the particular coral.  

9. The outside of the coral is more healthy than the inside

Many SPS corals grow in such a way that creates a tight network of branches, especially as they mature into colonies. This network of branches impedes water flow and is why you might notice the outside of a coral appear to be much healthier than the inside. The inside network of branches isn't receiving sufficient flow causing a decline in health. This is common in seriatopora hystrix (Bird's Nest Corals) but can happen to any SPS coral. 

10. Life, flow, and SPS 

As you increase PAR levels and light output to accommodate SPS coral, the metabolic processes within a coral speed up. This will increase the demand to export waste and toxins as well as intake nutrients and elements which means more flow.  The more light you give corals, the more flow they need to maintain optimal health. This is especially important for SPS corals because of their preference for high-light environments.

SPS corals

11. Flow really only matters at each coral

While surface agitation and stirring up detritus are important for the health of a reef aquarium, flow is most important when it comes in contact with corals. In other words, moving water in open areas of the tank where it does not come in contact with corals is essentially useless for the coral.

12. Flow is the solution to pollution

Water flow keeps fish waste and leftover food suspended where it can pass into your filtration and be easily removed.  Without sufficient flow throughout the display aquarium, this waste is left to settle among the rocks and down into the sandbed where it will slowly break down into nitrates and phosphates.

13. Water flow keeps the water oxygenated

Surface water movement is the primary source of oxygenation. Gas exchange happens at the water's surface so it is essential to always be moving the water's surface, constantly exposing new molecules to the air where oxygen can attach and diffuse into the water. 

When the power goes out, fish will quickly deplete oxygen levels unless you maintain surface water movement. This is why we recommend the use of battery backups or generators to supply temporary power to your powerheads and wavemakers during a power outage. Just a single powerhead can keep your fish alive, even for extended periods of time.  

14. Flow is not a brand

Achieving sufficient flow throughout your entire tank often involves the use of different types/brands of pumps. Each pump produces different flow patterns and each pump has its strength and weaknesses in terms of targeting certain areas of your tank. The most effective configurations for achieving sufficient flow often involve a combination of different pump types based on the aquascape and coral growth. Flow should never be an afterthought and is equal if not more important than light in terms of long-term coral health. Planning your aquascape around your pump placement is the most effective way to achieve optimal flow.