Flow is a vital component to a successful and healthy reef ecosystem and at times is not given the proper consideration that it deserves. We really feel that Flow can be even more important than lighting to a corals overall health. Regardless of the exact position of importance, it's no surprise that the big 3 are Lighting, Flow and Water Quality.

Flow can serves multiple purposes for your reef system. The right amount of flow can help keep your aquarium and water quality clearer and cleaner by suspending organic debris in the water column where they can be removed by predators and taken away by your overflow and filtration. Flow can help keep stony corals clean and free of unwanted detritus and algae build ups that can impact their overall health in both the short and long term. Flow also servers the critical function of bringing food particles in the water column to the hungry polyps and mouths of the corals in you reef tank.

It's easy to see why flow is so important, but there are a few considerations you should make when setting up your tank in order to maximize your success.

Quick Index

Return Flow vs Internal Flow


When we talk about Flow in our Reef Tanks we are often talking about total flow which is a combination of return flow and internal flow such as the additional flow created by flow makers rather than your actual return pump. When planning your reef tank, both types of flow should be considered independently as the both serve unique purposes. 

Return Flow

Return Flow is important for the overall health of your entire system which may or may not contain multiple different displays with varying amounts of internal flow. Regardless, all tanks benefit from increased and healthy return flow. The general recommendation for smaller tanks < 100 gallons is to aim for 5-10x turnover for system volume. This means if you have a system volume of 100 gallons, than you are going to want to size your return pump to deliver at least 1000gph at 6 ft head. That said, these are more guidelines than actual rules. If 3x turnover from your return pump is all you can muster than go for that. The minimum we would recommend is 2x turn over. Water movement gets increasingly difficult as the system volume increases. For example a 1000 gallon aquarium would require a 10,000 gallon return flow at 10x which can be difficult to achieve at certain head heights, especially with things like UV Sterilizers in line.

Internal Flow

This is the internal flow inside your display tank and is generated by wave makers and flow pumps. Popular pumps include neptune, ecotech, jebao, maxspect and others. These pumps usually come with gph (gallon per hour) ratings and can easily move 1000's of GPH.

The better, more expensive varieties have random flow patterns and programmable schedules which believe it or not can greatly increase the health of your reef tank. We recommend between 30x-90x internal flow. For example a 230 gallon display would require around 6900 gph of internal flow on the low end (or a couple of mp40s) and up to 21,000 gph (or around 6 mp40s) as might be the case in a large SPS or Acropora grow out system. 

When it comes to internal flow, often "more" of less powerful pumps would be preferred to say a single pump that just blasts the flow out. The reason being is that with multiple, weaker pumps we have much more pattern configuration options and positional flow control and just as with lighting less "hot spots" for flow. 

Almost no coral likes high direct flow (being right in front of a wave pump)

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Flow, Coral and Substrates: System Planning

The type of corals you choose to keep and the design of your system often affect the amount of internal flow desired. SPS Corals especially Acropora demand turbid, random flow patterns inside the display tank and a high internal turnover. Those types of systems will be good candidates for 90x internal turn over ratings although I would still recommend to start lower in the 40-50x range and adjust based on coral health and monitoring. LPS Corals on the other hand do not require as much internal flow. Especially things like Euphyllia (Torch, Hammer, Frogspawn), many meat corals (trachy, scoly, lobo), Duncans, Elegances, Leathers and other soft corals can be negatively impacted by too much internal flow.  Finding the right balance will take some tinkering and experimentation.

If you are wanting to setup a tank with high internal flow rates such is the case when designing an SPS dominant system we recommend using a larger particle sized substrate such as special grade sand and to stay from fine grain sand or fine substrates. Better yet, for high internal flow go bare-bottom with no substrate at all. When going bare-bottom other considerations will need to be made. We will discuss bare-bottom setups further in a later article.

When shopping for internal flow pumps, we recommend getting a pump that at least has some control over its pattern. Many cheaper off brand jebao pumps allow this type of control, as well as almost all of the more expensive, name brand varieties. The extent and ease of this control varies from brand to brand. Some of the best flow makers on the market include Neptune and Ecotech, but there are other great options as well.

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    Too much flow can be harmful to your reef tank inhabitants. Although polyps can become retracted and corals deflated for other reasons, flow is a good possibility to consider. A coral that is not extending its polyps or is deflated and/or retracted may be receiving too much flow. Let your aquarium speak to you and monitor it closely, if you see negative effects, make an adjustment. 

    An example: A while back I noticed that my leather corals in my 150 gal personal display do not like it when I run my Neptune wave makers along side my MP40s. Over the course of a few days, I noticed they became deflated and smaller/stubbier in appearance when all 4 pumps were running.

    My solution? To schedule the Neptune wave makers to only run for 10 minutes at the top of each hour.  This helps to lift detritus up and out of the tank, create a random increased amount of flow that changes the dynamics of the tank but only for a short time, and ultimately led to happy leathers and happier corals in general. This is just one solution and there could be other solutions such as changing the angle, type, and intensity of the flow, but continual monitoring of coral health can tip you off to any adjustments necessary. 

    People who ignore the importance of flow do so at their own peril!

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    Just as too much flow can be harmful to your reef, so can too little flow. Flow serves a critical function of bringing food to the hungry mouths of our coral polyps. Without high internal flow, many corals like SPS and some LPS that tend to have small polyps and little mouths with a limited ability for food capture will struggle greatly.

    Hard, encrusting stony corals like Acropora, chalice, platygyra, favia, montipora, cyphastrea and others often lack the means to clean detritus off themselves. They do not inflate or deflate, and therefor lack the movement that some other corals that like less flow have. If these corals get covered with sand or detritus it can be very deleterious to their health. Internal flow created by flow makers serve the critical function of keeping these corals clean, healthy and vibrant when planned and executed correctly.

    People who ignore the importance of flow do so at their own peril!

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    Unlike many other corals, Anemones can actually move themselves. Sounds great right? Well, not so much. Flow Makers can present unique challenges to keeping anemones in captivity. Consider that you just added a nice $250 dollar BTA into your main display. It's a big display 100 gallons+ and you have a few flow makers, maybe even on the low side doing about 2000-3000 gph each. 

    Everything seems fine at first, you put your BTA in the tank, you kill the flow while doing that so the anemone can safely attach, and after an hour or two you turn the flow makers back on. Things seem fine, your anemone is opening up and you're very pleased with yourself. You go to sleep. The next morning you wake up and UH OH, your anemone is stuck in the wave maker! This happens often with anemones in aquariums without proper protections in place. 

    In our experience and opinion, there is no safe flow maker for anemones. We have seen them get stuck in guards, many different types of guards have failed at one time or another, and ultimately the anemones tend to die if they make their way into the flow pump. 

    What is worse is that certain types of anemones can be much more dangerous in these situations than others. While a BTA who gets caught in the flow maker is not likely to wipe out a tank, a Haddoni carpet anemone on the other hand is very capable of wiping out your entire tank and killing everything inside it if it were to get caught up in a flow maker. Take it from me, I have personal experience. I kept the haddoni for over 4 months and it never moved, then one night..... Trust me, you do not want to be awake at 5am trying to untangle an 14" haddoni carpet from an MP40 wave maker while your fish are swimming into the walls of your aquarium and dying. Luckily I was able to save much of the tanks inhabitants by capturing them out and moving them to another system including the haddoni carpet. They were easier to catch due to their weakened state and I was able to act fast after the event took place. Still, I lost two of my favorite fish in a white tail bristletooth and a blue jaw trigger as well as my pair of captive bred mandarin's that I had kept for over 3 years. Still, I count myself as lucky for having happened to be awake at the time and hearing some undesirable sounds coming from the pump and had to live with the results. 

    It was a hard lesson that I learned. Two lessons in fact -- Lesson one: A 150 gallon display tank doesn't need an 14" haddoni in it. It's too big, and at that size is capable of wiping out the tank with its stinging cells. Lesson two: Plan your tank for your desired inhabitants ahead of time. Really think about it. Research it, and build it right. You do not want to be in the position of trying to adapt your already designed/established system to meet the needs of some new thing you've never considered before like a Haddoni Carpet.

    We strongly suggest hobbyists consider special planning if a large, show piece type anemone is a desired tank inhabitant. If that is the case, I would stick to corals that require very low flow, and try to get as much flow as possible out of the return pump. If done correctly, additional flow makers may not even be required which would be the ultimate solution for keeping a happy anemone. 

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    We like the ability to schedule our flow pumps. Smart pumps like neptune and ecotech allow great control over the flow pattern and minute-by-minute, hour-by--hour scheduling control. There may be other great options as well. Some cheaper options like Jebao allow for some varying pattern options, which is better than nothing, but often lack fine tune control. Still, they can be just as effective as the most commonly used pattern is a pulsating one and Jebao's handle this well.

    The benefits of scheduling include:

    • Higher flow for certain periods of time, such as 1 hour before the photo period beings and 1 hour after the photo period ends to help lift detritus even in low-flow LPS and softy systems. 
    • More random blasts of flow. Consider a cross flow wave pump that only turns on for 5 minutes each hour and blasts high continuous flow for 5 minutes. This can mimic tidal swells in the ocean and help to blast detritus away and keep it suspended while not impacting corals that do not appreciate the flow so much.
    • Keeping corals on their toes with flow patterns. Corals like random flow patterns that change and alternate through out the day. They do not like flow coming from the same direction in the same way all the time. The ocean does not do this, and we seek to emulate the ocean in all aspects when we are maintaining our reefs.

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    A Flow Maker is often only working as well as its last maintenance check allows it to. Over time with high pH and calcium in our reef tanks flow pumps can slow down due to the formation of calcium deposits and coralline algae. Certain brands seem to perform better than others over the long term, but a hobbyist should be inspecting these things at least once per month. 

    This does not mean that they must be serviced once per month, only that they should be checked on. If the pump does not seem to be performing as effectively as it was, maintenance is in order.  We typically maintenance our pumps by submerging them in a mixture of water and vinegar or water and citric acid. Acids like vinegar and citric acid do a good job of killing back anything that may be impeding the pumps performance. After a good hour-long soak we scrub the flow maker with a tooth brush thoroughly, disassembling it if we can. After which we rinse in RODI water to clean away the acid before returning to the system. 

    It is important to rinse the pumps after you clean them, otherwise the residual vinegar can have a nasty impact on your tanks pH for a short period of time. Checking on wave pumps should be done as often as possible, but most pumps will only need the acid bath treatment once every 3 months, of course this can also depend on the cleanliness of your aquarium.

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