Marine Refugium Plants: How a Separate Growing Chamber Cleans Your Water
The first time I actually watched my refugium working I was astounded by how effective it was. This thing just tucked away in my sump full of green spaghetti-looking stuff was quietly removing nutrients from my reef tank twenty-four hours a day. No need to change filter media or add chemicals. Just let the plants do plant stuff underwater…
If you’ve struggled with stubborn nitrates or bouncing phosphate levels, adding a refugium might be just what your system needs. It’s an ultra-effective form of biological filtration taken to its logical extreme. Rather than depending solely on bacteria to process food waste and fish bio-load into inert substances, you’re introducing macroalgae to devour those nutrients and convert them into biomass you can physically harvest out of your system.
The concept is simple enough it almost sounds too good to be true. But the basic science is well understood, and real-world results have been proven by countless hobbyists who just can’t imagine going back to trying to manage nutrients any other way. My refugium dropped my nitrates from a frustrating 15 ppm to less than 2 ppm in about six weeks. And more importantly, it kept them there.
## Understanding Refugium Filtration On A Biological Level
**Nutrient Export By Removing Biomass**
This is the big one. Macroalgae take up nitrate and ammonia as they work to improve water quality (Petco). When you harvest the algae and physically remove it from your aquarium, you’re exporting those nutrients with it. That isn’t recycling nutrients like other filtration methods. It isn’t converting the nutrients from one form to another. You are simply removing them from your system entirely.
To give you an idea of how cost-effective this method is versus others, nitrogen removal research out of Harvard places prices at £1 – £16,000 per kilogram depending on the method used (Harvard DASH). Macoalgae biomass harvest effectively removes nutrients from your system (AlgaeBarn), making refugium filtration one of the cheapest available methods when compared to commercial options for reef tanks.
**Reduced pH Swing Through Photosynthesis**
A helpful but often unexpected side effect of running macroalgae in your aquarium is reduced pH swings. This is because macroalgae consumes CO2 from the water column when it photosynthesizes, causing pH to increase. At night when the algae switch to respiration they release CO2 back into the water lowering pH. The right setup can actually reverse your pH swings.
Run your refugium lights opposite your main display tank lights. 12 hours on / 12 hours off creates an ideal reverse photoperiod (Reef2Reef) that allows your macroalgae to photosynthesize while your corals are respiring and vice versa. Without this your pH swings will be far worse. In one example someone actually documented a swing from 8.29ph to 8.03ph. That’s a variation of 0.26 units (Reef Central).
This will actually happen in your system without a refugium too. Your corals/photosynthetic fish and macroalgae are all competing to raise pH during the day. At night they’re all lowering pH just as quickly. Refugium macros help soak up that excess CO2 overnight.
**Synergy With Your Existing Biological Filter**
A refugium chamber isn’t intended to replace your bioload processing systems. It’s there to assist your existing solutions. You will still need sufficient live rock to handle the waste produced by your fish load. Aim for 1 to 2lbs per gallon of live rock to properly process bioload while maintaining refugium export (Reef Tank Resource). Any waste that exceeds the capacity of your live rock biofiltration will be handled by your refugium.
## Building Your Refugium Grow Chamber
**Location And Water Circulation**
Refugiums can mean simply having a portion of your tank dedicated to macroalgae on your display rock. However we’re going to focus on refugiums that utilize a separate connected chamber in your sump (AlgaeBarn). Design and placement of that chamber are important considerations.
Water flow through the chamber is a careful balance. You don’t want water moving so fast that your macroalgae get torn up. But you do need enough movement to deliver nutrients to the algae and bring oxygenated water to all parts of the refugium.
This was a problem for me when I first set things up. I foolishly put my refugium chamber directly under the return pump outlet. I thought more flow = more nutrients delivered to the macroalgae. What actually happened was choppy water movement that constantly tumbled apart my chaeto making fragments that were sucked up into my return pump and dispersed throughout my display tank. You want slow but steady movement. Aim for laminar flow vs turbulent flow.
**Selecting Refugium Substrate**
Your substrate can make a difference both for what algae you decide to grow and what else happens in the refugium chamber. Many people utilize a fine aragonite sand bed to anchor their macroalgae and provide beneficial bacteria surface area to process detritus. About 2-3 inches of sand is plenty deep.
I’ve also successfully ran refugiums with zero substrate. You give up some of the benefits of beneficial bacteria that process food waste when using sand, but you make maintenance and harvesting a lot easier.
**Lighting Schedule And PAR Requirements**
One of the biggest mistakes people make when setting up a refugium is trying to use “leftover” lighting or running the same lights they use for coral husbandry. The algae you’re growing in your refugium have vastly different requirements than your SPS corals. Most refugium macroalgae don’t require intense light or high PAR (Reef2Reef). Chaetomorpha grows well at PAR values of 100-200. That’s less than half of what many corals need. Affordable LED strips produce ample light output.
Your lighting schedule is just as important as the intensity of light you’re running. Like with the pH stabilization benefit above, your refugium lights should be on while your main display tank lights are off. 12-16 hours is typical (Bulk Reef Supply). To further reduce pH swings run your refugium lights opposite your display tank. If your tank lights are on from 8AM to 8PM, try 8PM to 8AM for your refugium.
The overlap actually isn’t a big deal. I run mine on what I call a reverse photoperiod. My tank lights are on from 10AM to 8PM, while the refugium lights are on from 8PM to 8AM. That leaves about 2 hours of overlap in the evening where both are on, and another two hour period in the morning where both are off. It creates a more gradual transition than switching from all lights on to all lights off.
## Choosing Your Macroalgae Species
**Chaeto: Grow Easily, Grow Often**
Chaetomorpha has become almost ubiquitous amongst refugium keepers for good reason. It grows like a champ, deals with a wide variety of water parameters, won’t escape your refugium to takeover your tank, and it’s easy to harvest. Chaeto resembles a green scouring pad, growing into a tight ball that you can literally scoop handfuls out of once it gets big enough.
It does require ample flow to prevent detaching and flowing out to your display tank. It also can be picky about lighting. Too little and it will start to turn white and eventually die. Too much flow and it will constantly be torn up and disaggregate. Find the happy medium and chaeto will reward you with fast growth.
**Caulerpa: Faster Growth With Known Risks**
Fastest grower of the three primary macroalgae choices. Caulerpa can actually take over your entire system if it escapes so many people use chaeto for that reason alone. It grows in beautiful thick bushes that provide copepod habitat and are very attractive.
Caution is required when keeping Caulerpa due to potential for them to “go sexual.” Think of it like them randomly deciding they don’t want to live in your tank anymore. They’ll dump toxins into your system and start dying off rapidly. If you have a huge refugium and it does this, you can quickly crush your entire tank. Can happen to chaeto too, but much less frequently.
**Red Macalgae: Slower Growing but more Stable**
Gracilaria is my goto “back up” macroalgae. It grows slower than both chaeto and Caulerpa, but it’s also much less likely to just suddenly die for seemingly no reason. A more stable refugium addition.
## Mistakes To Avoid When Adding A Refugium To Your System
Miss Take #1: Over-sizing your refugium for your system. Your refugium can only process as much bio-load as your tank is generating. Too big and your macroalgae will begin to turn pale and growth will slow considerably. Match your refugium size to your feeding habits and fish loads, not the amount of space available in your sump.
Mistake #2: Using the same lighting schedule as your display tank. This absolutely wrecks your pH stability as all of the photosynthesis and respiration is happening at the same time throughout your system. During the day everything is photosynthesizing causing pH to rise. Overnight everyone is respriing and pH plummets. Reverse photoperiod tricks your system into spreading these processes out.
Mistake #3: Never Harvesting Your Macroalgae. This kind of goes hand in hand with #1 but deserves its own bullet. If you let your macroalgae grow without harvesting it will eventually use up all available nutrients then start starving. Once it begins to starve nobody wins. Algae needs to be harvested regularly to effectively export nutrients. Once every two to three weeks is ideal. Think of removing 1/3rd of the total biomass each harvest.
Mistake #4: Expecting Instant Results Macroalgae doesn’t grow overnight. Even once established it can take several weeks to reach maximum growth rates. You may not see changes to nutrients for 4-6 weeks. During this time continue performing regular water changes. A refugium isn’t a magic cure-all.
Mistake #5: Inadequate Water Circulation Areas of stagnant water will develop in your refugium over time. Likely where your bulk of macroalgae is located. Waste will collect here and macroalgae will suffer. Flow doesn’t need to be extremely strong. You just need it to reach the entire surface area of your refugium.
Mistake #6: Not paying attention to stable salinity levels in your refugium. Like corals, macroalgae don’t respond well to sudden changes in salinity. Make sure your top-off system is keeping your reef at proper salinity targets of 35 ppt and 1.026 SG(The Salty Side) everywhere. Including your refugium.
## References And Sources Used In This Guide
Some links in this guide are included to help back up points made in the article with scientific studies, firsthand experience from other hobbyists, or simply product/material references. Links that provide supporting documentation for the points made (**bolded**) above can be found below.
Each is cited somewhere within the context of the article but having them in one place will make it easier to find if specifics are needed. I’ve tried to use peer-reviewed sources where possible, especially for specific data like nutrient export levels and pH fluctuation levels before/after refugium integration.
Studies into refugium systems are hard to come by but we do have decades worth of evidence collected from the hobby itself. The how and why macroalgae improve water quality has been studied extensively. Nutrient uptake rates have been established as have lighting, flow, and many of the nuances involved in successfully operating a refugium.
Because refugium gardening borrows from hydroponics and macroalgae aquaculture we can also draw from hundreds of years of commercial research into these fields. Much of this data crosses over into our own homes as we try to recreate successful freshwater and marine systems on a smaller scale.
Refugium cultivation itself is straightforward enough that much of the research into tuning specific parameters for ideal growth has been crowd-sourced by millions of hobbyists growing algae for fish food or skin care products. When you couple that experiential knowledge with real-world science from institutions like Harvard and the EPA it’s easy to see why refugiums can be so effective.
## Refugium Macroalgae Options For Different Aquarium Types
Nano Reef (20-40 Gallons)
Even aquariums this small should have room to dedicate at least 10% of total system volume to refugium filtration. Nano tanks have limited space and bio-load. Chaeto is probably your best option until you get a feel for your system’s capabilities.
Your goal should be establishing the refugium and maintaining consistent macroalgae harvests. Don’t worry about growing as much algae as possible. Balance bio-load with refugium size.
Standard Reef (50-120 Gallons)
Sweet spot for refugium use. Big enough to put together a properly proportioned refugium chamber in your sump. Enough fish/ bio-load to properly feed your macroalgae. Shoot for a 20-25% refugium volume for these aquariums.
Over 150 Gallons
Continuous harvesting may not be necessary on once-per-month intervals depending on system size. Multiple refugium chambers or very large chambers work well on these systems. Fill the void with different macroalgae species. If chaeto crashes due to instability try adding gracilaria or Caulerpa to the mix. Different species will react to water parameter fluctuations differently.
Heavy Feeders
All those pods flying around your refugium aren’t just there to look pretty. They also happen to make an amazing supplemental food source for your fish. Refugiums are ideal for heavily stocked aquariums or systems with large fish that create lots of food waste. Continual nutrient export will prevent the spikes that cause algae outbreaks.
SPS-Centric Systems
If you are chasing 0dKh/0dPh water conditions a refugium may seem counterintuitive. For good reason! Macroalgae require nutrients to grow, and SPS-only systems often don’t produce enough waste to sustain large refugium setups. Use refugiums on SPS reefs with caution. Raise your nutrient export target levels and keep a close eye on nutrient levels after each harvest. Symptoms of stress from low nutrients in corals can resemble algae crashing in your refugium.
Fish Only Marine
Refugiums absolutely can be used in freshwater aquariums but they truly shine in saltwater setups. FOWLR aquariums without coral enjoy higher nutrient levels and don’t have fragile creatures that limit the types of algae you can safely keep. Grow fast growers like Caulerpa without fear of hurting your fish.
## Refugium Benefits Beyond Nutrient Export
Less Frequent Water Changes
A refugium wont completely replace the need for water changes. Saltwater mix is still expensive and frequent water changes still ensure proper trace element levels are maintained. That being said, a properly sized refugium can reduce how often you need to change water from once a week to every other week or even monthly in some cases.
Better Coral Growth/Royal Blue Coloration
Stable water parameters and less algae competition generally leads to healthier coral. The reduction in pH swings alone can have profound impacts on SPS corals and their ability to calcify.
More Pod/Living Food
A refugium can become it’s own ecosystem of sorts. As mentioned previously refugium macroalgae become a magnet for copepods, amphipods, and countless other microfauna. These organisms then become food for everything else in your tank. Additional hiding places and living foods can lead to healthier, more aggressive feeding response from your fish and corals.
Cost Savings
Less chemical media means you won’t need to replace charcoal, phosphate remover, or other filter media nearly as often. Water changes once every two weeks instead of weekly can save you a fair amount of money on salt mix as well.
Failsafe
Your refugium doesn’t just replace aspects of your existing filtration setup, it enhances them. If your protein skimmer decides to die on you or your weekly maintenance falls by the wayside your refugium continues to work double-time keeping nutrients in cheque until you can get things sorted.
Natural Solution to Algae
Certain types of algae are never going to be completely avoidable. Diatoms, hair algae, and even cyanobacteria (blue-green bacteria) are all naturally occurring forms of algae that every aquarium deals with. Refugiums can help limit large algae outbreaks by competing for available nutrients. Limiting spikes in nutrients from heavy feeding days keeps algae at bay.
## Budgeting & Planning For A Refugium
Phase 1 – First 2 weeks
Get your refugium chamber established and add basic lighting. An inexpensive LED strip light and a bit of chaeto can begin improving your water quality right away. Use this time to plan out your upgrades from here.
Refugium Chamber – Already have one as part of your sump
Basic LED Refugium Light – £30-50
Chaeto Culture Start – £15-20
Timer (make sure it can do a reverse photoperiod) – £10-15
Total Cost: £55-85
Phase 2 – Weeks 3 – 6
Your basic lighting should be setup but can be upgraded to improve macroalgae growth. Add substrate if you didn’t get around to it in phase 1. Keep a close eye on growth rates and adjust harvest frequency based on actual conditions in your tank. Macroalgae growth will vary based on a number of factors.
Full-Spectrum LED – £60-100
Sand Substrate – £15-25
Better timer with multiple programs – £20-30
Total Cost: £95-155
Phase 3 – Week 7 – 12
By now your macroalgae should be well established. Make any changes to improve circulation if you noticed dead spots during phase 2. Consider adding more macroalgae species to diversify your refugium and experiment with tuning parameters. Take detailed notes on growth rates, nutrient levels before/after harvest, and any problems you experience. Tracking your own data lets you fine tune your system beyond what generalized guidelines can provide.
Small Refugium Circulation Pump – £25-40
Additional Macroalgae Species – £20-35
Testing Kit (if you don’t already have one) – £25-40
Total Cost: £70-115
Grand Total: £220-355
One of the benefits of setting up a refugium is you don’t have to do everything at once. Start small and build your setup over time. By setting goals and having a plan it won’t feel like you spend all your money upfront just to watch your tank for month. After everything is said and done you’ve got a system that will continue to give back. No Filter media to buy, no additives to buy more of. Feeding your fish will be expensive, but just about everything else that goes into water quality maintenance becomes optional with a refugium.
