How Does a Refugium Work?

How Does a Refugium Work?

A refugium is an auxiliary tank that is connected to the “main” tank. While the main tank is used mostly for display, the refugium serves mainly utilitarian purposes. In short, refugia naturally turn unwanted materials such as dissolved and particulate wastes into valuable live foods, namely “pods” (especially copepods) and macroalgae.

Some aquarists are satisfied with the positive results and leave it at that; some are more curious and wonder, how exactly do refugia make this magical transformation?


To understand how refugia work, one must understand how primary production and primary consumption work in Nature. Primary production is carried out by autotrophic plants and bacteria, which generate harness energy and synthesize biomass from inorganic sources of carbon and nitrogen (most often through photosynthesis). Primary consumers simply are the organisms that eat primary producers (e.g. herbivores). Primary consumers therefore transfer food energy and biomass from plants/certain bacteria to carnivores. As such, primary producers/consumers form the very base of every food pyramid!

Macroalgae take up excesses of dissolved nutrients (e.g. nitrate and phosphate) that commonly build up in closed systems such as aquaria. In so doing, they compete with less desirable benthic microalgae (e.g. film-forming or filamentous types). Whatever nuisance algae do grow can be controlled by copepods, which are naturally abundant in almost all marine ecosystems. In addition to “bad” algae, copepods (and many other pods) consume detritus.

Now, it’s great if your aquarium animals eat your pods and macro--these are nutritious, natural live foods, after all! But in the tiny confines of an aquarium, these food sources can get exhausted quickly; hence
the refugium! Refugia provide a comfy refuge where the pods and macro can get pampered, safe from the hungry jaws of your livestock. The pods simply drift into the main tank little by little, whereas the macros can be offered as a treat whenever you harvest (to keep the macroalgal bed in a state of constant growth!).


So, that's the biological side of how a refugium works. How about how a refugium physically works? How do you implement a refugium into your system? There has been a greater exploration of how to add a refugium to several different types of aquariums so that people with smaller aquariums or those without sumps can experience the benefits of refugiums as well.

The most common way hobbyists add refugium to an aquarium is by dedicating a large section in the sump for the refugium. A large portion of commercial sumps intentionally incorporate space for a refugium that is separate from the protein skimmer and reactor chamber. However, just because this is the most common method, it doesn't mean there aren't other methods that work with other tank styles. Some sumps don't have space for a refugium. In this instance, you can add a separate tank that is connected via a manifold or an additional pump. A refugium can be just as large as the whole sump or even as large as your aquarium.

What about tanks that rely primarily on hand-on-the-back (HOB) filtration? Well, there are HOB refugiums. Some are incredibly cheap and simple, while others are more premium and expensive. It works the same as other HOB equipment works. This method could actually be used on a sump as well.

Some aquariums are built in what is often referred to as the "all-in-one" style. This term can also refer to tanks that come with the sump and stand. So, to be more specific, we are talking about tanks that include a filtration partition in the back of the tank. This is common in nano aquariums, but it can also be found in large aquariums as well. For a nano aquarium, the best method to add a serious refugium is with a HOB refugium. For large all-in-one style aquariums, you can use one of the sections in the back of the tank as a refugium. A lot of these aquariums have a section intended to be used for bio-balls. However, you can use this section for a refugium instead.

Hobbyists often wonder (and argue) about where in the filtration system a refugium should go. Should it go before the skimmer? After? Like many things in this hobby, there is no definitive answer. However, it is encouraged that you put the skimmer first and the refugium second. There are several reasons for this.

  1. It allows both the refugium and the skimmer to do their jobs better. If the refugium goes first, a lot of the organic material that has not yet been broken down tends to get caught in the refugium. This means that instead of the organic materials being removed by the skimmer, they are sitting in the refugium and turning into nitrate and phosphate.
  2. Having the refugium seconds means there is less detritus buildup, allowing the macroalgae to do its job of removing nutrients better.
  3. Having the refugium first results in copepods and other beneficial microscopic crustaceans having to get through a skimmer before they can get to the return pump. If you don't care about maintaining your copepod population to feed a finicky fish (or for any other reason), this is not as important for you. Regardless, it is another reason refugiums may work better if they are the final stage of the filtration process.
Of course, it all depends on your goals and how your tank is set up. This topic is highly opinionated, but those are some reasons to run a refugium after a protein skimmer instead of before. In the end, it is not that important, but still something to consider. What is more important is the lighting you provide your refugium.


How does the lighting in a refugium work? Macroalgae, like nuisance algae, are photosynthetic organisms. They need light to grow. However, the lighting requirements of macroalgae are different from corals. It is certainly possible to grow macroalgae under reef lighting, but to reach maximum growth and export from the macroalgae, it is best to use refugium specific lighting. Algae tends to grow better under red dominate lighting with a bit of blue. There are lots of different ways you can run your lighting. Some hobbyists run a short light period, others run a long one. Some run the lights the same time as the display lights. The most common way to run refugium lights is the opposite of the display lights. This is typically done to stabilize pH levels.

How do you get the lights to run on this schedule? Well, it depends what light you have. There are all sorts of refugium lights in all budget ranges. Some mount above the refugium, while others stick to the side with the light shining through the glass. A lot of the more premium lights such as the AI Fuge Light can be scheduled with an app or on board programming. Others such as the Innovative Marine ChaetoMax, require a timer or aquarium controller to control the schedule. Obviously, being able to control the light from your phone is the most ideal, but those lights are typically more expensive. If you can only afford a cheaper light, you may still be able to reap amazing benefits from your refugium and macroalgae! It all just depends on your specific tank and what you want to do with it.

Determining which lighting is best for your situation greatly depends on the size of the refugium and the lighting in the display. For a lot of hobbyists, having a refugium and/or macroalgae is for competing with algae in the display. This means that you need a light that is going to grow algae better than the light in your display. If you have intense lighting in your display, you are going to need fairly strong lighting for your refugium. The ChaetoMax light is generally better for smaller refugiums, while some of the higher end Kessil lights will work great for large refugiums. AlgaeBarn sells a large array of different refugium lights at different price points. That way you can pick what is best for your tank.
That about sums up how a refugium works for a saltwater aquarium. Remember to do what you need to do to maximize the functionality of the refugium. With a refugium working together with the other components of your filtration, you will see many benefits to your tank!
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