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2020 Guide for Optimal Water Flow and Turnover in a Coral Reef Aquarium

2020 Guide for Optimal Water Flow and Turnover in a Coral Reef Aquarium

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Mike Paletta Reef Tank — At least part of the success of this tank is that despite its calm look, water flows in it at over 80 times the tank's volume per hour.

Pop Quiz: If the power goes out and you can only run a few pieces of equipment on your aquarium, which do you select first?

Lights, right?

WRONG! Your corals can survive for four days or longer without the lights being on.

But if you allow the water in your tank to be still for just a few hours, your corals will begin to show signs of distress. A rapid decline will follow if the situation is not rectified.

I, and many other authors, have been advocating for strong water movement in reef aquariums for the past 20 years. Some have even written that flow may be more important than light when it comes to growing healthy corals.

Over the years, I have learned many valuable lessons. One is that I rarely lose a coral because it is receiving inadequate light. But I have lost many corals due to improper or insufficient water flow.

Detritus that needed to be basted off the frags daily when inadequate flow was present.

A Short History of Water Flow in Reef Aquaria

In the early days of the hobby, flow simply meant how big of a pump is being used to move water from the sump to the tank and back again. Then, powerheads were added along with wavemakers and surge devices. Finally, Gyres and smart apps to change the flow patterns throughout the day and night.

While most of us realize that a reef is not like a river or a pond, few of us realize just how much flow there is in and on a reef. Now, I do not mean wave-crashing flow. I mean flow that when you try to remain stationary in one spot to look at a coral or take a picture, it requires all your effort to stay in that spot type flow.

Sadly, despite most hobbyists understanding that there is a lot of flow on a reef, there is not nearly as much discussion about flow as there is on lighting, nutrient levels, or even what to feed a tank. So, if we are going to want to improve the flow in our tanks, we need a better understanding of just why it is important and what optimizing it will do for our corals.

Flow does not need to be elaborate just adequate in order for a tank to thrive.

Why Water Flow is So Important to Corals

First, corals are sedentary. As a result of being stuck in one place, it is necessary for food, nutrients, and other useful material to come to them. Flow determines if and what food comes to a coral since it can’t move to capture its food. This is also the case for nutrients. If there were no flow, the coral would quickly use up the food, nutrients, and structural elements surrounding it. It would cease growing once these were all used up.

Flow removes the waste corals produce along with waste products and detritus that settles on and around the colony. If the flow is inadequate, detritus can build up around the coral, which may allow harmful bacteria to thrive that can attack the coral.

Lastly, when corals spawn, flow moves the gametes away from the mother colonies and, hopefully, to adequate places for settlement. Most importantly, flow is essential for driving photosynthesis and respiration, two critical processes for coral survival. Because so many factors related to flow are critical to a coral’s health, it is easy to understand why having adequate flow is so important to have healthy corals. In my experience, inadequate flow can lead to detritus build-up, nutrient accumulation, and thin, brittle corals.

Most LPS corals do not require the flow of SPS or soft corals, but it should still be strong enough to keep detritus from settling on them.

Four Different Types of Water Flow

This leads to two questions:

What is adequate flow?
How can it be produced in a reef tank?

Flow, or water movement, can be classified in four different ways:

Turbulence
Surge
Laminar
Gyre

Each of these types of flow are produced in different ways, and each has a different effect on corals.

Turbulence

Turbulence is when random water movement results from when water comes into contact with either another current of water or a solid object. In most of our tanks, we have microturbulence. This happens when the flow from two powerheads comes together. It also occurs when the flow contacts live rock or corals and, as a result, changes its direction. It is one of the best types of water movement as it keeps dead spots from occurring and keeps things moving around the corals in a random pattern.

Surge

Another type of flow is called surge. In this type of movement, water moves back-and-forth in a semi-constant pattern. If you have a soft coral or gorgonian tank, this type of movement can be observed when the polyps or blades move one way for a while and then switch and move back the other way.

Surge movement can be produced by having powerheads placed at opposite sides of the tank and alternating when the pumps turn on and off. In large aquariums, this type of movement is produced by what’s come to be known as the Carlson surge device. This device was developed by Bruce Carlson when he was at the Waikiki Aquarium. The device consists of a large dump bucket that is gradually filled with water. Once full, it will tip over and dump all of its contents at once back into the tank.

When designed properly, it produces an impressive surge in the tank along with excellent oxygenation and turbulence. The downside is it also tends to produce a lot of salt creep. It also produces so many air bubbles after it’s dumped that visibility into the tank is diminished for a while. A Carlson surge device requires a fair amount of maintenance and, as a result, is only useful on the largest of tanks.

Laminar

The type of flow that is most familiar to aquarium hobbyists is laminar flow. This type of flow is what we see when water shoots out of a powerhead. It is unidirectional and the easiest type of flow to get in an aquarium. It is also, in my opinion, the least useful of the four types of water movement discussed in this article. Even when a powerhead is connected to a device that moves it around, the flow is less random than the other types. If the pump is too strong, it can knock corals around or even shear off coral tissue. If the pump is too weak, it only moves water around in a limited area.

Gyre

The last type of flow we will discuss is gyre, which is relatively new to the aquarium hobby. Gyre flow moves in a robust circular motion and moves a great deal of water with it. Jake Adams began talking about Gyre flow almost 15 years ago, but only in the last few years has the technology caught up in the aquarium industry to produce it.

This type of water movement is produced by “gyre” pumps. One is placed near the top on one side of the tank with another on the bottom of the opposite side of the tank. The pumps move a large volume of water across a large cross-section. With flow going across the top on one side and on the bottom of the other in a circulator patter, a “gyre” is created. Gyre pumps move far more water than any other type of water movement system. As a result, anything that gets caught in the Gyre: food, detritus, waste, etc., stays in the water column for an extended period of time.

When set up properly, the flow in a soft coral tank can make the corals move to and fro just as they would on the reef.

How I Define Adequate Flow

Knowledge of all these different types of flow and how they interact and affect corals has lead me to set specific goals when designing and implementing optimal flow in my systems.

First, I want the flow to remove as much detritus as possible from around the corals themselves. I also want this flow to keep the detritus in the water column for as long as possible because I want it to be removed from the main display into the overflows and sump so that mechanical filtration and protein skimmers can remove it. This not only keeps water moving across my corals, but it also moves a lot of water on the bottom of the tank.
Second, I want the flow to be somewhat random so that no dead spots occur.
Third, I want the flow to be controllable. That way, when I feed my fish and corals, I can slow the flow down so that it is not immediately pushed out of the tank.
Fourth, I want the flow to produce surface agitation so that CO2 can be driven from the tank and replaced with oxygen.
I should add that this last goal requires more than just water moving devices. The CO2 levels in the room where your aquarium is located need to be controlled as well. Many of us keep our homes sealed in the summer and winter months. In doing so, the CO2 levels climb to levels that impede the exchange of CO2 from our tanks with oxygen from the air around them. As a result, the pH of our aquariums declines which inhibits the growth of some corals. I’ve written an article all about CO2 that will be published here at CoralVue very soon. In the meantime, just keep in mind that water movement alone cannot fix poor gas levels.

Just as on the reef, anemones do best when in moderate to strong flow.

How Water Flow Effects Lighting

Another aspect of flow, particularly surface flow, that must be considered is how much flow affects lighting. On the surface, it may not seem like there is much of a relationship at all.

Flow affects light levels in two ways:

First, if the flow stirs up a lot of detritus, this can reduce light penetration, especially toward the bottom of the aquarium.
Second, as water moves across the surface of the tank, it creates waves. These waves have peaks and troughs, just like they do on the surface of the ocean, so-called “glitter lines.” These peaks and troughs act like little magnifiers and diffusers and affect how strong the light is penetrating the water. That is why if you use a PAR meter in a tank with strong surface agitation, you will see it fluctuate as the peaks and troughs move across it. However, if you turn off the water motion, you will get a steady number. In my opinion, having vigorous movement across the top of the tank is positive in this regard as well, since it keeps light from becoming too focused at any one time.

Even in this small tank, the flow needs to be adequate. Here it is 100 times the tank's volume.

I Just Need a Bunch Of Powerheads, Right? Wrong.

Since we’re trying to get as much flow as possible, then all we need to do is throw as many powerheads into the tank as possible, right? Problem solved!

Not so fast, Silver Surfer.

Sure, more powerheads will produce more flow, but they will also create more heat and noise. They are also unsightly. You’ll have wires dangling everywhere. Not to mention they all require regular maintenance.

If the goal is to create a massive gyre or turbulence, the pumps need to be coordinated. They also can’t be all laminar flow. Producing too much of this can shear tissue off corals if it is too intense. Lastly, if the tank is not bare bottom, then concessions need to be made so that the substrate is not blown about and over the corals.

Here is the foaming action when a Gyre pump gets cleaned. Please wear gloves and protective eyewear while cleaning your pumps. While the reaction is not violent it, can cause irritation.

How I Increased Water Flow by 50% in My Reef Tank

I have been playing around, manipulating, and trying to optimize the water movement in my aquariums for the last 20 years.

I have used dump buckets (too noisy and dangerous), closed-loops (leaks and failed pumps), eductors (backpressure killed the pumps), wavemakers, and just about every combination of powerheads and controllers imaginable.

On my new 600-gallon system, I thought I had planned for adequate flow. But, after 6 months, I added to it and redesigned it, increasing water flow by 50% to better meet my corals’ needs.

Initially, my goal for this tank was to create as much random flow as possible across the corals with additional strong flow across the bottom. I attempted to create a minor gyre, flowing from side-to-side across the tank. I had strong flow focused across the top that moved the water toward the opposite end of the tank where the overflow to the sump was located.

Unlike most tanks, the overflow in this tank covers the entire wall of one of the short sides. My plan would allow the flow to move detritus more easily out of the tank. The flow was initially created by three EcoTech Marine MP60 pumps across the top on one short side of the tank, and three Tunze Stream 3 pumps on the bottom of the opposite side of the tank. I also placed three Tunze 6155 controllable powerheads and used a Sicce 2500 gallon per hour return pump. The total flow throughout the tank was approximately 80X the tank’s volume (my actual display tank is 500 gallons). I thought this was more than adequate.

However, over time it became evident that despite this relatively high flow, the tank’s flow needs were not being met as detritus was settling along the bases of most corals. Since most of the corals were frags, this allowed bacteria to thrive in these areas and in some instances, it killed the small frags. There were also dead spots on the bottom and detritus was simply not staying in suspension long enough to be removed as I had envisioned. Because of this, I realized I had to up the flow rate in the tank and improve its design. I should note that in my experience, the flow in larger tanks (in terms of it times the tank’s volume) needs to be greater than it would be for smaller tanks. Frags and frag plugs also tend to accumulate more detritus around their bases than larger colonies.

To increase and improve the flow pattern, I made several changes. The three Tunze Stream 3 pumps were replaced by Panta Rhei ECM 63 pumps. I added two more EcoTech Marine MP60 pumps. I replaced the Sicce return pump with an EcoTech Marine Vectra L2 pump. I also removed the Tunze 6155 pumps.

I also purchased a HYDROS WaveEngine and connected two Maxspect Gyre 280 pumps and two Maxspect Gyre 350 pumps to it for power and control. Now I was able to create a true gyre-type flow. These additions increased the flow so that it was now approximately 120X the tank’s volume—roughly a 50% increase.

There were other benefits, too. Having the Panta Rhei pumps on the bottom on one side and the Gyres powered by the WaveEngine on the other produced the kind of gyre I wanted. Detritus was in suspension longer and the ORP increased by 50, which to me is an indicator of cleaner water. It was also evident that the new flow was getting more detritus out of the water. I went from having to replace the filter socks every 4-5 days to every 2-3 days.

The fish pecking at the Gyre when it is shut off.

The Average Turnover for the Best Reef Aquariums is 85X

When my tank was first set up, I thought there would be more than enough flow. I actually thought the 100X the tank’s volume was high. However, recently I collected data from 20 hobbyists who I feel have outstanding aquariums with healthy, rapidly growing corals. The turnover rate in their aquariums ranged from 40X to 120X the tank’s volume with an average of 85X. Based on the numbers I recorded, raising my own turnover to 120X was by no means unprecedented.

Getting Rid of Cord Clutter with the HYDROS WaveEngine

Adding the WaveEngine and four Maxspect Gyre pumps was a great idea. They were very easy to attach and program: I just watched a video tutorial on YouTube! Now, all of the Gyres function in a coordinated fashion. They are easy to shut down when I want to feed the tank, The flow is also automatically reduced at nighttime.

The WaveEngine has done its job by making water flow more manageable. One of the benefits of the WaveEngine is that it allowed me to get rid of four power bricks and four pump controllers, which otherwise would have required space for mounting and a lot of wire management to keep organized. Now all of the pumps are powered and controlled by a single device all in one spot.

I know I can add a second WaveEngine since they communicate wirelessly, but I would love it if they released a larger model so that all eight of my pumps could be managed from a single source!

The wall of Gyres powered by the WaveEngine on one side of the tank.

The Gyre Pump Trade-Off: More Flow, More Maintenance

The Gyres are outstanding for their size. They move more water than any other device I know of. The flow is not so strong that it shears the tissue off nearby coral. Fish also don’t get sucked into them like they do with traditional laminar flow pumps.

They are a bit of a pain to clean, though. So every three months like clockwork, I remove them from the tank for cleaning. I coat them in baking soda and rinse them in white vinegar. I follow that up with some gentle brushing and a rinse in freshwater. This removes most of the grunge. A full disassembly once a year for a deep cleaning is something I intend to do, like I do annually with most of my equipment.

If flow is inadequate, dead spots like this can become problematic.

A New Rule of Thumb?

When I set up this tank (really a system of tanks), I took over a year to plan everything. Except for water flow, I haven’t had to alter or replace many items.

A good rule of thumb many hobbyists and I follow is that when there is a problem in the tank, a water change will usually help. In all my years of reefing, I’ve discovered another rule of thumb. And that is when there is a problem I just can’t put my finger on, increasing the flow in the tank usually helps. In my opinion, flow is only a problem if it is too intense. Otherwise, it’s not a bad thing to have “too much flow.”

With all the improvements and innovations available to us, it is easier than ever to create adequate flow. Even in large reef aquariums like mine, so that fish and corals behave just like they would on the reef.