How Big of a Refugium Do I Need?

When adding a new refugium to your aquarium setup, one of the first questions that’s probably going to pop into your head is, “How big does it need to be?”. I wish that there was a refugium size calculator or even a straight-forward answer to this question, but just like many other subjects in this hobby, there isn’t one. Not all is lost though, my friend. I’ve designed this article to get you on the right track, and at least one step closer to achieving your refugium dream.

To start things off, I’ll explain the standard advice given to aquarist regarding the size of their refugiums, then explain how such advice is flawed and how more thought needs to be put into it to design and implement a successful system. After that, I’ll explain all the important components of a refugium and how they need to properly work together to make the system effective.

To finish things off, I’ll showcase some popular refugium solutions and explain the benefits of each of the two large categories of refugiums that you will see on the market. If you’re more of a DIY fella, not to worry, I’ll end the article with a step-by-step video on how you can start constructing your very own in-sump refugium at home.

Common Advice for Refugium Size

If you have searched online, asked a friend, or talked to a rep at your local fish store for advice regarding the required size of a refugium you should install, an answer that you probably got was anywhere from 10-50% of the display tank’s water volume. Although this may be true to some extent, it definitely isn’t the end-all be-all information that you should be relying on when designing and installing a refugium in your system. There’s just too much more to account for.

What Really Matters in a Refugium

For a refugium to be effective, there are three elements that need to be considered:

  1. The amount of nutrients you needs to reduce
  2. The strength of light you use
  3. The amount of contact time your aquarium water has with the refugium macroalgae

To form a solid grasp on why these are the three important things that matter when installing an efficient refugium, let’s review the basic idea of how a refugium works.

The Basic Idea of a Refugium

To start the process off, water is passed into your refugium from your display tank. While in there, it flows through a macroalgae organism (most commonly chaetomorpha). The macroalgae is just like any plant species in the fact that it utilizes the light that is shined on it and the phosphate, nitrate, and carbon dioxide in the water to grow, develop, and produce oxygen through photosynthesis.

As as result of this whole process, the phosphate and nitrate in your aquarium water decreases, the carbon dioxide in your aquarium decreases, and the concentration of dissolved oxygen increases. These are all great benefits for you aquarium and the life that call it home.

One of the great benefits is that it keeps the phosphate and nitrate levels low, which is important not only for the wellbeing of your aquarium inhabitants, but it also reduces the growth of algae in the display aquarium. Another great benefit is that it reduces the drastic pH swings that aquariums experience when the lights go out.

You see, it’s common practice to light the refugium on an opposite cycle relative to the display aquarium. By doing this, carbon dioxide doesn’t build up overnight because the refugium macroalgae is hard at work converting it into oxygen through photosynthesis. By keeping a more stable carbon dioxide level like this around the clock, pH tends to follow suit.

There are plenty of other benefits when it comes to the topic of refugiums, but for the scope of this article, I believe that I have explained their functionality and purpose well enough to continue.

Learning the Theory

As you can imagine, there are quite a few factors that can deviate from refugium to refugium, and those factors are going to dictate the efficiency of the refugium once it’s implemented. For example, if you’ve got a system that produces quite a lot of nitrate and phosphate, the larger your refugium should be to complete the job. Now having a larger refugium may not take care of the nitrate and phosphate if the light that you install on the refugium to grow that macroalgae isn’t powerful enough.

On the other hand, you also have to account for the amount of contact time that the aquarium water has with the macroalgae as it passes through the refugium. If you have a very fast water movement throughout your refugium, the less time that the macroalgae will have to strip the water of its phosphate, nitrate, and carbon dioxide before the water re-enters the display tank.

To learn a little more about these topics, I recommend watching the great 3-part refugium experiment conducted by Bulk Reef Supply, listed below.

Let’s Talk Products

It’s practically impossible for me to recommend an exact size of refugium, the amount of macroalgae you should put it in, the perfect lighting system, and the proper pump to keep the water flowing through it at the ideal refugium flow rate. There are just way too many factors that change from system to system.

What I can do is point you in the right direction by showing you a few of the different options that are out there today. Since I’ve already given you a solid foundational knowledge of what refugiums are, how they work, and what you need to look out for, the only next steps is for you to find out what works for your unique aquarium system and continue the journey.

With that being said, there are two general categories of refugiums that are available on the market:

  • Hang-on refugiums
  • In-sump refugiums

Hang-On Refugiums

Hang-on refugiums are just that – a refugium that hangs onto the rim of your aquarium. With this style, water is pumped from your display aquarium, up an intake tube, and into the refugium reservoir. As the water level raises within the reservoir, the water is inevitable poured back into the display tank.

These types of refugiums are very easy to access, which is definitely a good thing when you have to routinely remove and replace the macroalgae as it grows too large. The one downfall to these types of refugiums, however, is that they can be an eyesore for some aquariums. I don’t know about you, but I prefer to have the least amount of equipment hanging on the rim of the aquarium as possible. I just like the look of a more natural-looking system. With that in mind, however, this may be the option that is most suitable for your system.

If I were to recommend any hang-on refugium out of the ones that are available today, I’d recommend the one from Finnex. Their model is a decent size and comes with everything you need to get started – the pump, casing, and even an LED light. They’re not too expensive, so if it’s your first take on adding a refugium to your tank, it might be a good option to test the waters.

In-Sump Refugiums

The most popular place to have a refugium is in the sump of your aquarium. Just think about it, you’ve already got the water moving from the display aquarium through the filtration media and pumping back to the display aquarium.

Yes, by adding the refugium directly into the sump, the refugium will be a little harder to get at when you have to remove some of the macroalgae as it becomes overgrown, but it gives you the pleasure of not having to add any new equipment to be seen on the display aquarium.

Another added benefit of having the refugium in the sump itself is that you don’t need to add any more pumps to your system to get the water from the display tank into the refugium. By not adding any additional pumps, your not adding any equipment that can potentially fail and need to be replaced.

And finally, by adding the refugium away from the aquarium, you’ll be able to light the refugium freely without any of the light entering the display aquarium, disrupting the internal clock of your fish and potentially causing algae growth within the display aquarium. All good things.

How to Build a Refugium Sump

Below I have included a video from one of my favorite YouTube channels, The King of DIY. In it, he walks you through the process of building a sump. Obviously, this one 4 minute video isn’t the only one I would watch if I were you and was going to build a refugium for the first time, but I hope that this is a good starting point for you to become educated on how a sump works and how they can be used for setting up a refugium.

That’s All Folks

Throughout this article I have explained the theory and processes of refugiums, gave you some idea on what size of refugium you will need, as well as the amount of light and refugium turnover rate you will need to account for too. I went through the benefits of the two general categories of refugiums and even gave you a few specific products that you could start out with. I hope that throughout this article you have become a more knowledgeable fishkeeper and that I have completed my job in assisting you with getting one step closer to your refugium dream.

Till next time.