This 180-gallon display tank has 15,000 gallons per hour total tank turnover.

This tank belongs to, and the photo is courtesy of @fabutahoun, ©2019, All Rights Reserved.

This is an article that I’ve been wanting to write (or get someone else to write) for quite a while. Tank turnover is something that is not well understood by beginners, and there is a lot of confusion about it. Furthermore, beginners are often reluctant to ask questions about topics that they perceive that everyone else understands.

So, in this piece, I’m going to attempt to define what tank turnover is, why you need water movement in a saltwater aquarium, and how to achieve it.


What is tank turnover?

Tank turnover means different things to different people.

Definition #1:

If you have a sump, then for some people, tank turnover means how much water flows out of and back into your aquarium through the sump. This could also be called sump turnover. And let me interject here, that just because you have a pump that is rated for 1000gph, for example, that doesn’t mean that 1000gph is actually flowing. The actual amount of water flowing into and out of the display tank will be reduced by friction and how much height the pump has to overcome—like say it has to pump up vertically 15 feet from your basement. I’ll save a discussion of pumps for another article.

Definition #2:

If you do not have a sump, then for you, tank turnover may mean just how much water is circulating or moving around in your tank. In this case, you add up how many powerheads you have and what each one is rated for. You could also theoretically add to this number if you have a skimmer (and how much water it is processing) and any other mechanical filters that move water (and how much water they are processing). You add up all those gallons per hour and you have your total tank turnover in gallons per hour or whatever unit you’re using.

Definition #3:

The last definition is the one that most people are thinking of when they talk about tank turnover. This is simply #1 + #2 or how much water is going out of the aquarium and coming back in (if you have a sump) plus how much water is moving around inside the aquarium. Some people refer to this as tank “flow.” As in, “how much flow do you have?”

So, now that we have a vague idea what tank turnover is, let’s talk about why we need it. If you have experience with freshwater aquariums, you probably didn’t worry too much about how much flow you had.

Simple diagram of display tank and sump (below display tank). Water moves in a clockwise direction, gravity-fed down on the right and pumped up on the left. The small blue box is the pump.

Diagram was furnished by @sbash, ©2019, All Rights Reserved.

Why do we need flow in a saltwater aquarium?

There are a lot of reasons why you need flow in a saltwater aquarium. Here are some of them, not in any particular order:

1. Gas exchange.

Movement of the water helps with gas exchange. It helps to have carbon dioxide or nitrogen gas released to the air and oxygen to be added to the water, for example. Oxygen enters the water at the surface or is mixed with the water in a skimmer. Flow helps all parts of the tank to have equal oxygenation.

Cold water can hold more dissolved oxygen than warm water, and fresh water can hold more dissolved oxygen than saltwater. This is one reason why freshwater aquarists don’t fret about dissolved oxygen like reefers. The warmer and saltier your water is, the less oxygen it holds.

2. Movement of ions.

In the same way that water flow increases the chances of equal oxygenation throughout the tank, water flow also ensures that you have an equal spread of other needed ions or nutrients. For example, barely moving corals are taking up calcium around them; flow of water replenishes what they use up faster than if there was no movement. Oxygen radicals are also moved away from corals, as too many of them in the vicinity of coral can be detrimental.

3. Suspension of detritus.

Movement in the water keeps junk aloft, and that’s how the garbage travels through the filter and/or skimmer and/or sump to be removed in some kind of filter like a filter sock or your skimmer. Water quality is extremely important in a reef tank because any detritus that sits around will break down eventually lowering the water quality.

4. Feeding livestock.

Lots of livestock doesn’t move much, like a Tridacna clam, for example. The ability of this type of livestock to eat is a function of what and how much and how often something edible floats past it. Yes, clams also have zooxanthellae that help feed their hosts through the use of sunlight, but they are also filter feeders. And filter feeders need something to pass by to filter.

5. Removing waste from livestock.

For livestock that doesn’t move much, the motion of the water also removes their waste and washes it away, well as much as you can wash something away in a closed captive reef. But the water action also serves other purposes such moving away irritating chemicals or toxins that neighboring livestock may produce.

6. Water movement is what the livestock is used to.

As much as possible, reef aquarists try to recreate a little reef. Real reefs have water movement and sometimes lots of it. So, if you’re try to make your livestock feel at home, so to speak, then you have to give them water movement.

7. Avoiding “dead” areas.

Experienced aquarists frequently speak of “dead” zones and how they are accidents waiting to happen. Having flow everywhere in the tank will help you to avoid problems like with cyanobacteria and nuisance algae.

So, now that we’ve covered why you need water flow in your reef tank, what kind of flow do you need?

Well, that’s not an easy question to answer. What you want to avoid is a lot of laminar flow or constant steady flow in one direction only. You want something that’s chaotic and/or surge-like and/or with an ebb and flow—like the tide. You probably wouldn’t enjoy your garden hose with a narrow, tight, power-wash spray setting, spraying on your arm in the same place 24/7, and your corals don’t like it either.

And how much flow do you need?

And now we get to the interesting part. It depends on who you ask, and also what kind of livestock you have, and whether you’re talking about internal water movement or total tank turnover (see #3 earlier). So, let’s take a look at these variables.

If you have a sump, then you can run as much water through it as you want, in theory. Although if you have a skimmer that’s only processing a small amount of the flow, then you don’t gain a lot by having a kiloton of water pass by per hour. Yes, maybe that water is all flowing through a filter sock, but is the water that dirty? And yes, maybe you’re oxygenating it—a little—but the part going through the skimmer is oxygenating its part a lot more.

So, for the sake of discussion if you have a 100-gallon display tank and a 50-gallon sump, and you anticipate a heavy bioload, let’s say you choose this skimmer (I just picked one—I’m not married to this one.) The max flow-through recommended for this skimmer is 260GPH.

Screenshot of Reef Octopus Skimmer rated for 150G well-stocked.
Screen Shot 2019-03-16 at 4.19.32 PM.png

Screenshot courtesy of @Seawitch from the Bulk Reef Supply website, ©2019, All Rights Reserved.

That means that for in/out tank turnover, one could argue that anything more than 260GPH, if you run the skimmer wide open, is a waste. And you probably don’t want to run it wide open. Let’s say you want to run it at about 200GPH. Your display tank (100G) + your sump (50G) = 150G Total Tank Volume*. So, you could really run as little as 1.3X your tank volume in gallons per hour through the sump. You could run more water through, but your skimmer won’t be touching it.

Edit: If you read the comments below the article, @ca1ore in comments #4 and #9 pointed out that 1.3X your tank volume is probably not enough to run through the sump and that most people keep their heaters and dosers in the sump, so it's preferable to run more than I suggested. Furthermore, skimmers are not as efficient as I suggest, and it's impossible to be sure exactly how much water is being processed by a skimmer. I stand corrected.

You can run 300GPH through the sump or 600GPH or 6000GPH. I don’t care. My point is that you don’t have to get all your needed water movement through the sump because you can increase your water movement inside the display tank with powerheads. But you can run a lot of water through the sump if you want to, and that will cut down on your need for creating additional water movement inside the display tank.

So, let’s ask the question again: how much total water flow do I need overall for my display tank?

Again, it depends on who you ask. Interestingly enough, I found a great video on the topic by Bulk Reef Supply. In that video, BRS says they recommend 20-50X tank volume total turnover.

And who needs a low- or medium- or high-flow tank? In general, the rule is (and rules are made to be broken) soft corals need low flow, large polyp stony coral (LPS) need medium flow, and small polyp stony coral (SPS) need high flow.

A 190-gallon reef (soft coral and LPS) tank with about 2000 gallons per hour tank turnover.
r2rtankturnover (1).jpg

This tank belongs to, and the photo is courtesy of @Pntbll687, ©2019, All Rights Reserved.

So let’s come back to our example of the 100-gallon tank with the 50-gallon sump. Let’s say you want to aim for a medium-flow tank at 30X total tank turnover or 30 X 150 gallons = 4500GPH.

If you run 200GPH through the sump, that leaves 4300GPH you need to move inside just the display tank. Maybe slightly less if you don’t count the sump. So, how do you do that? You add some powerheads that move that much water, like four (4) powerheads that each move 1000GPH or eight (8) powerheads that move 500GPH or eight (8) that move 1000GPH and run them each ½ time to create some chaotic flow. You feel me? You can do anything you want.

You can also run a mechanical filter to move water. Yes, you can. Purists are going to flip out when they read this because a mechanical filter can create a host of problems if you don’t clean it a lot, like every other day. But you can have one. Not everyone has a sump and a filter sock.

The reason you have to keep a mechanical filter relatively clean is because any junk or detritus that the mechanical filter catches will break down over time and cause your water quality to deteriorate. A dirty filter becomes what is called a nutrient sink. But if you’re vigilant, and you rinse out the detritus a lot so it doesn’t have time to break down, then the mechanical filter A) performs the same job as a filter sock, B) moves water and adds to your total flow, and C) it will also become colonized with beneficial bacteria that will help with chemical filtration.

A mechanical filter will never perform chemical filtration as efficiently as live rock, but the filter can be a plus and not a minus.

In another good video, this time by Marine Depot, they said they recommend 10-20X tank volume total turnover for a “low flow” tank, 20-40X tank volume total turnover for a “medium flow” tank, and > 40X for a “high flow” tank.

Then I watched a good video from Vivid Aquariums. In that video, “low flow” is defined at 10-15X turnover, “medium flow” is 15-17X turnover, and “high flow” is 20X turnover. Hmmmmm.

As you can see, this is not an exact science, and I’ve heard of people having much lower flow through a sleepy but thriving refugium and much higher flow, like 100X total tank turnover, in a display tank. In fact, the top photo with this article shows a tank with almost 100X tank turnover, while the 2nd tank photo shows a tank with about 10X tank turnover.

I’m going to stop here, and in Part 2 I’m going to talk about what different types of powerheads have to offer and how to decide what you need. We’ll also hear from some forum members on what the flow is in their tanks.

*I'm doing these calculations in terms of total system volume (display tank + sump, if you have one). You can also reason this through in terms of just the display tank volume, as long as you understand that overall you have more water in your "system" than just the display tank, for purposes of dosing, for example.



We encourage all our readers to join the Reef2Reef forum. It’s easy to register, free, and reefkeeping is much easier and more fun in a community of fellow aquarists. We pride ourselves on a warm and family-friendly forum where everyone is welcome. You will also find lots of contests and giveaways with our sponsors.


Special thanks is due today to forum member @sbash who is helping me with creating diagrams.


Author Profile: Cynthia White

Cynthia received her BA in English from NYU a long long time ago. She has been a freelance writer and editor for over 20 years. Now she is a writer and editor on staff at R2R, where her forum nickname is Seawitch.
  • Like
Reactions: esther