A Complete Guide to Flow for Reef Aquariums

What is flow in Saltwater Aquariums

Flow in a reef tank or FOWLR aquarium is the amount of water, in gallons, that is moved per hour by a piece of equipment. Flow is important is because it helps remove gases from the water that would otherwise compromise the health of fish, corals, and invertebrates. When fish breath, they take in Oxygen and exhale Carbon Dioxide. The CO2 that is exhaled will build up in the water and eventually affect the chemical balance in the tank, including PH and alkalinity. This change in PH and alkalinity will eventually have a negative affect on the life in your marine aquarium. Another benefit of higher water flow is nutrient transport. Higher water flow moves waste and rotting food from the sand and rock to the water column and surface. When brought out, filters and overflows can pull this waste out of the tank, helping to manage nitrates, and lead to a stable reef aquarium.

If you want to understand more about how flow in your reef tank is important, check out this video from Marine Depot.

How much Flow for FOWLR Tanks?

Fish only with live rock (FOWLR) aquariums require far less flow than reef aquariums as there are no corals, and the fish are not as sensitive to the PH changes. The fish in FOWLR tanks can generally tolerate parameter swings a better than corals, permitted there is enough flow for the gas exchange. Most beginners to the hobby will start with a FOWLR tank and eventually add corals, which frequently die or do not thrive as expected. Unfortunately, when doing this, there is often very little thought to upgrading the wavemakers and powerheads for better flow in the saltwater tank. If this is the case for you, I recommend starting with the higher flow so that the tank does not need to undergo changes later.

What is the Minimum Turnover Rate for Saltwater Aquariums

Turnover refers to the number of times the entire water volume of your aquarium is moved. If you have a 50 gallon aquarium, and a powerhead with a flow rate of 200 GpH (Gallons per Hour), the turnover rate would be 200GpH / 50G = 4 times per hour. Now that you know how to calculate turnover rate in the aquarium, what is the minimum amount needed? The minimum recommended turnover rate for a FOWLR aquarium is between 10 and 20 times per hour. The needed flow rate for a reef tank is (size of tank) x (turnover). For example 50g x 20 = 1,000gph. Therefore, if you have a 50 gallon aquarium, you should aim to have at between 500 and 1,000 gallons per hour of flow at a minimum.

Though you may be able to get away with 10 times turnover, I strongly encourage you to aim for 20 times turnover. Aiming for the higher end of the "minimum" or "low" flow rates gives you a safety net in the case of equipment failure. Other considerations include obstacles such as rocks or decorations in the way of the water flow, or even just in case of too many fish leading to larger amounts of gas exchange needed. Just don't get too carried away. If the flow is so high that the fish can't swim, it's too much.

This all sounds great, but like many other areas in the saltwater aquarium hobby, there are some exceptions. Specialty tanks, like jellyfish or seahorse tanks will need less flow, closer to the 5 to 10 times turnover. The low flow requirements is because these fish are poor swimmers and high flow will lead to injury or death of the fish. Other fish that are more common, such as puffer fish, are also poor swimmers, but they can handle well over 10 times turnover. With poor swimming fish, I suggest that you do some research and determine what flow is best for the fish. Remember that longer tanks will also have less flow in the center than directly in front of the wavemaker, meaning you may be able to have the best of both worlds.

How much Flow for Reef Tanks

I want you to take a second now, and check, what is the turnover rate of your aquarium? How much flow do you have? I think it is important before you continue reading to know where you currently stand. Are your corals thriving? Growing faster than you thought was possible? Or just surviving? Okay, do you have the number? Let's continue.

Reef aquariums have additional gas exchange, nutrient transport and other flow related needs. These needs strongly depend on the types of corals you want to keep in your reef tank. Proper flow is one of the most important components, besides lighting, to success or failure when keeping coral, especially SPS and LPS. This is because soft corals (softies), Large Polyp Stony (LPS) corals, and Small Polyp Stony (SPS) corals all have different water flow needs.

Flow Rate for LPS and Soft Corals

Every species of coral is a little different and you should do your research before purchasing and placing your corals in your reef tank. Most soft and LPS corals will do best in low to medium flow reef tanks. This would be anywhere between 20 and 40 times turnover. If you choose more flow, these corals should be placed out of direct flow from pumps, wavemakers, and powerheads. If the corals are closed or not doing well, then try to reduce flow rate, or move them to a lower flow area, blocked by rock or other obstacles in your reef.

Flow Rate for SPS Coral

Similar to soft and LPS corals, every species of SPS coral is different and you should do research before adding these corals to your reef tank. Most SPS corals will do well between 40 and 100 times turnover. This wide fluctuation in flow is mainly due to the different variations of coral. Acropora, for example, usually need higher flow rates than some birdsnest corals. If the polyps are not extending, it is likely that this coral needs more light or water flow. SPS corals frequently like being directly in the water flow and rarely have issues with too much flow, unless placed too close to a powerhead.

Types of Flow

Laminar or Constant Flow

Laminar or constant flow is usually created by powerheads and pumps. Laminar flow is not the preferred form of water flow as there is no changes or movement, leaving some part of the reef tank without flow, known as dead spots. This is an area where nutrients can build up and become a source of nitrates. Whenever possible, it is best to avoid constant or laminar flow. Though in some cases this cannot be avoided. One example would be the return pump from the sump. Quarantine tanks can usually get away with laminar flow as it is not as critical when corals are not present.

Gyre or Oscillating Flow

Gyrating flow is a back and forth motion, usually created by a small surge that will bounce off the opposite wall of the aquarium, and then be pushed back again. This type of flow will create large amounts of force in back and forth motions and will help create a fair amount of movement in the tank, and aid in less dead spots and better nutrient transport. This type of flow is much better than constant flow sources like that from powerheads. This oscillating flow is usually created with a gyre or oscillating powerhead. Though there may be other ways, the gyre will be the best source of this flow.

Turbulent Flow

Turbulence is the crashing of water flow to create randomized flow patterns. This random pattern leaves nearly no dead spots, and being random it will move waste. This is the best flow for coral health as it will best aid in nutrient transport. Turbulent flow can be quite difficult to get in your reef aquarium without specific wavemakers. I personally recommend the Vortech MP10 and MP40 by EcoTech as they do a great job at creating these flow patterns.

Though not as random or strong as the Vortech, turbulent flow can be created a few other ways. One way to create turbulent flow is by cycling powerheads on and off frequently and with "random" or different on-off times throughout the day. Another option is combining multiple oscillating flows, at different rates. The combination of both of these would lead to even more turbulence and random flow. However, by the time you purchase the equipment and put in this amount of work, you would likely be better off to purchase the higher end wavemakers.

Conclusion:

Determining the Best Flow for your Reef Aquarium

Using the above information, it is best to create turbulent flow utilizing an EcoTech Vortech MP10 or MP40, or at least utilize a gyre for changing water flow patterns and reducing dead spots in your tank. The turnover, or flow rate is heavily dependent on the type of tank as well as the fish and other inhabitants you want to keep in the aquarium. Using the above information, you should be able to determine the best flow for your saltwater fish tank.

If this article was helpful to you, please share it and help someone else that is looking for help determining the best flow rate for their aquarium.

Recommended Reading:

• Benefits of Bare Bottom Fish Tanks
• Ecotech Vortech MP40 Review
• Saltwater Aquarium Lighting Guide

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About the Author

John Krenzer

John is a Software Engineer with a passion for saltwater aquariums, as well as the founder and president of Reef Stable. He started in the aquarium hobby as a child with a 20 gallon freshwater aquarium. His interest in aquarium life grew and in 2008, John set up his first saltwater aquarium.

Today, John maintains an over 300 gallon reef tank system, consisting of a 120g reef and a 210g reef. These large tanks are contained within the same system, sharing a sump as a means to reduce total maintenance and increase total water volume.

John writes articles for the blog as a means to learn about more reef aquarium topics. These articles act as a reference for the readers as well as himself. John updates these articles frequently to provide additional information or make corrections as new information becomes available.

If you would like to request an article, tank tour article, or to colaborate, let me know via the Contact Me Page!

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