Water Flow in the Refugium by greg.chernoffWhen it comes to setting up your refugium, there are plenty of considerations to make. You must select macroalgae species, clean-up crew species, a type of substrate (or none?), lighting and so on. Most of this planning is pretty fun. But, with so much to think about, and so many options, it might be easy to overlook one of the most important factors of all: water flow. Water flow characteristics can significantly impact macroalgal growth rates and, therefore, the refugium’s overall capacity for nutrient export. And controlling dissolved nutrient levels is one of the main reasons for installing a refugium in the first place, right? Types of Water Flow The first distinction aquarists typically make when discussing water currents is whether they are chaotic or laminar. Chaotic flows are most like those encountered over shallow reefs that are subjected to strong wave action. As the term suggests, chaotic flows are highly inconsistent with regards to direction and velocity. Laminar flows are more like those in rivers and estuaries, as flow direction and velocity are more or less constant (they may change direction with the tides, but are nevertheless comparatively stable and predictable). [Refugium] So what does this look like in a refugium? A chaotic flow pattern might be created with the use of specialized water pumps. Usually controllable to some degree, these pumps can be adjusted for variable output, for movement (of the nozzle or the entire unit), or for both. In creating chaotic water movement, it might help to place the inlet from the main tank in the center of the refugium so that it may progress in all directions. Laminar flows are a little more straightforward to create. In this case, you’ll want to direct the inlet to the far end of the refugium opposite the outlet. The water will simply flow from one side of the refugium to the other. Generally, for generating laminar flow, supplementary pumps are not necessary. Which to Use? Type of flow is usually dictated by the kind of macroalgae to be kept. Velocity can be higher for rigid forms (e.g. Chaetomorpha), but must be subdued for softer, fleshier types (e.g. Ulva). In more special circumstances, this will also be additionally influenced by the type of animals that might be housed there. For example, when using a refugium as a place to keep seahorses, a relatively gentle, laminar flow is ideal. Most seaweeds will grow either attached or unattached. But, many prefer to grow one way or the other. Laminar movement that flows through the crop are best for attached macroalgae. If the aquarist aims to cultivate types that form canopies (e.g. Ulva) or sheets (e.g. Chaetomorpha), laminar flows are likewise suggested. This is most easily accomplished with a longer refugium. Chaotic flow can be used to tumble the macros and is highly effective with unattached macroalgae. This is most easily accomplished with a taller refugium. The reason for tumbling is that it is incredibly efficient, since all parts of every plant receive light. These kinds of currents are most appropriate for compact, globular macros that don’t readily form canopies or big, solid masses (e.g. Gracilaria and Halymenia). Lighter, fleshier algae such as ogo require only a moderate flow rate, whereas heavier, denser algae such as leafy sphere appreciate something more vigorous. How much flow is enough flow? Really, if our primary purpose is to remove nutrients, the higher the turnover of water through the refugium, the better. There are, however, some physiological limitations here. First of all, there will be some maximum efficiency at which the macros are capable of taking up the nutrients. And then there are also physical limitations to how much flow can be applied. This is because the more delicate types like Ulva and ogo can be damaged from shearing or compaction in the face of strong currents. If one chooses a softer macro (maybe because the intent is to feed harvested material to herbivorous livestock), an “oversized” refugium is a good idea. Why? Because you can pass more water through it even at a reduced velocity! Settling Things There is an interesting caveat to the more-is-better rule for velocity. In terms of nutrient sequestration, yes, you want an overall faster flow. Still, it is quite desirable to have small pockets of lesser flow here and there. The reason for this is that it allows suspended particulate matter to settle out onto the bottom of the refugium. This accomplishes two things. For one, it reduces detritus build-up in the display tank (where you really don’t want to see it). Secondly, this deposits it right where your sludge-eating bacteria (e.g. PNS Probio™) and detritivorous harpacticoid copepods will be most abundant. The macros themselves help with this. Friction slows water currents as they hit the plant surfaces. But one can fiddle around with baffles, the rockwork, pump orientation and so on to maximize settling. Going With the Flow No two refugia are exactly the same. A single refugium might even change considerably between growth/harvest cycles, as it biologically develops, etc. Therefore, unfortunately, there is no handy formula for the perfect flow pattern. But with some awareness of how water movement affects macroalgae growth (nutrient sequestration rates!), any aquarist can make adjustments as needed to get the most out of their planted refugium. [Refugium]
Robert Blevins says March 29, 2019 at 4:40 pm I think less flow through the fuge is best for several reasons. Reply
Edwin Boston Jr. says May 6, 2019 at 5:37 pm I have a medium flow and it keep things in really good shape. Reply
Chad Sosby says May 6, 2019 at 7:44 pm My mandarin that I got from you guys would eat anything. Reply
Rudy Brambier says May 7, 2019 at 8:44 am With proper use of wave makers or circulation pumps in the display tank you can lower the flow threw your refugium to maximize your filtration. Reply
kevtide says August 24, 2019 at 7:41 pm I’m not sure I’m following. When you say lower the flow through your refugium, does this imply that you ramp down your return pump speed and increase display tank flow through the use of wave makers, etc? Reply
Ricky J Gronwall says May 9, 2019 at 7:19 am These are great tips. Haven’t had a fuge in a while but setting up another soon! Reply
Jack Goldstein says May 12, 2019 at 6:15 pm My caulerpa has definitely grown faster since I added a small powerhead. Reply
Victoria Brewer says May 13, 2019 at 4:54 pm Great info I’ll have to try playing around with the flow to see if it affects growth Reply
Douglas Campbell says May 14, 2019 at 6:39 am This info is absolutely amazing for the new hobbyist and for the experienced one as well just need to take the time to read and apply the information Reply
Diana Aliprandi says May 20, 2019 at 7:13 pm Thanks for the article. There’s so much stuff to learn about the refugium Reply
Carol Mauch says May 20, 2019 at 11:24 pm Liked the information about the MA in regards to the amount of flow different kinds prefer. Fun to learn new things to add to the basics. Reply
Kevin R Gravier says May 21, 2019 at 12:19 am I appreciate the good info to take into account for the many facets of making a successful refugium. Reply
Dwayne Newell says May 21, 2019 at 1:46 pm Turning up my return pump has force more water to circulate through the refugium allowing the Chaeto to take off Reply
Jose Ponciano says May 27, 2019 at 11:41 am So do I need a wave maker I. The refuge??? But flow is very important.. Reply
Robert Vice says May 27, 2019 at 5:07 pm maybe I need more flow I use the eshopps adv series and half the water goes through the fuge unfiltered and the other half goes to the skimmer after passing through the sock just switched to sea lettuce and its growing good Reply
mcc says May 28, 2019 at 9:47 pm Good information. I’m interested in building a display refugium. Reply
Danny DJ says June 3, 2019 at 4:44 pm Still battling if an external algae scrubber is better than a refugium. I think I’m going with the scrubber tho. Thanks for the information. Reply
Kenneth Showman says June 3, 2019 at 9:52 pm I found a lot of valuable information in these articles to consider when I get my refugium up and running. I know my refugium will be a better success because of the willingness to help educate the reefing community. As always the Algae Barn has the best high quality products with the right knowledge to go with them. Reply
Garrett says June 16, 2019 at 2:49 pm Great information, I have Laminar flow through my sump. Where the water passes through a battle into the refugium is where I placed my Chaeto, so all the water passes straight through the macro. I have seen great growth after moving it into the direct current of flow. It used to sit tucked in the corner opposite the water baffle and wasnt receiving much flow. I wasnt getting much growth either. Now it seems to be loving the new location. I have it placed as some would use filter floss or pads. Reply
William says July 23, 2019 at 8:42 pm Can anyone recommend an ideal water flow rate (gph) for both chaeto and sea lettuce in the same refugium? Reply
Ed says July 11, 2020 at 12:25 pm Very interesting, thanks for this info here. I’ve run a hob fuge with a dsb on a 90 gallon reef years ago & it worked great. I’m getting back into the hobby & going nano with the Fluval Sea Evo 13.5. I have a small CPR refugium going to hang on the back, now I think I’ll try using the tanks built in mini sump for faster flow macros, live rock, etc & the cpr hang on I plan on a very slow flow using a continuous siphon instead of any extra pump for it & that will flow through miracle mud with some mangroves. I also have a mini skimmer but I don’t really think I’ll need to use it with this set up. Reply
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