You may have heard the term refugium if you’re in the aquarium hobby, but do you know what it is and the benefits it can bring to your set-up? A refugium is essentially a safe area for inverts and macro algae, but it also acts as a biological filter to help control nitrates and phosphates. Refugiums may be part of your sump, a separate hang on back unit, or even another tank plumbed into the display tank system. Generally, it consists of a deep sand bed (DSB), live rock, and macro algae with very slow water flow through the area and a relatively strong light source to support the live plants and inverts you choose to put inside. The light should be in the spectrum between 5,500-8,000k to allow for proper photosynthesis.
Let’s take a look at the different styles of refugiums first. One of the most common types is the in-sump refugium. This is a chamber in the sump that has a slow flow (roughly 30% of the water flow from your overflow box) moving through it. This flow rate allows the macro algae and beneficial bacteria to remove waste and nutrients. The best in sump refugium designs are set-up so you can control the flow through the unit, like Trigger Systems Ruby Elite. This style of refugium tends to be one of the most efficient.
Hang-on-the-back refugiums essentially work like a hang-on-back power filter. A motor pulls water up into the container where it passes through the live rock and macro algae, then slowly cascades over a lip back into the tank. This particular type of refugium is great for smaller tanks in regards to filtration, but on larger tanks, the primary purpose is to grow and maintain certain food sources for certain fish, such as copepods for dragonettes and macro algae for tangs and other herbivores. You can One make one of these on your own for a small tank using an Aquaclear 110.
Many marine aquarists choose to create a refugium tank that is plumbed into their main system. These are generally slightly more effective than the other types because they tend to be larger and have a lot more surface area for filtration. Depending on the space you have, you could place a substantial amount of live rock, sand, and macro algae in such a vessel. It can even serve as a secondary display tank, housing different species of macro algae and certain species of filter feeding inverts such as Xenia, Goniopora, certain anemones, tube worms, and sponges. The actual tank is plumbed into the main system via the sump or an overflow, and would have an adjustable slow flow like the others to allow the algae and filter feeders to pull nutrients and waste out of the water as it passes through.
Occupying Your Refugium
Once you decide on the style of refugium that suits your needs best, you’ll need add the necessary ingredients for successful biological filtration. Start with a deep sand bed. This is an area of sand or aragonite between four and six inches deep where beneficial bacteria can colonize. These anaerobic bacteria convert nitrates into nitrogen gas, completing the nitrogen cycle.
Placing several pounds of live rock or rubble in your refugium will also be beneficial in the same way as it is in your display tank. The rock aids in biological filtration and it will create a space for live occupants of the refugium to attach and hide.
There are many different species of macro algae that can be used in a refugium set up, but probably the most common types that you will come across in the aquarium trade are Chaetomorpha and several species of caulerpa. Chaetomorpha, commonly known as Chaeto (pronounced kay-toe), has a wiry appearance and texture, and grows in a ball-shaped mass. Chaeto grows relatively quickly, and it does not release spores into the system. As each mass grows and you will find that there are a lot of different species of small invertebrates that will take refuge in the coarse strands. Caulerpas are common in the aquarium trade. There are quite a few different species of Caulerpa, probably the two most common being those with feather-shaped leaves and the grape type that looks like bubble or grape clusters growing on the strands. Caulerpas also grow relatively fast, so they’re great at consuming nutrients. Keep in mind that may Caulerpas “go sexual” periodically spreading spores and possibly finding its way into your main tank. Caulerpas should be trimmed regularly to prevent this, and some people swear that having light on it 24/7 also prevents the action. Despite the risk, fast growth makes them a great sustainable food source for herbivores in the main tank, as well as very efficient biological filters. Remember, if you choose to keep macro algae in your refugium, it will need an adequate light source to thrive.
Mangrove seedlings have also become common and popular refugium additions in the past few years. If you’d like to keep a mangrove, you have to make sure you have enough room above your refugium for growth and a very strong light. Mangroves don’t really do much for nutrient export, because they are a very slow growing plant, but they make for an interesting addition to a refugium nonetheless. They require daily misting to remove salts from the surfaces of the leaves.
Caulerpa racemosa image referenced from wikipedia and originally posted by Nhobgood
Chaetomorpha image referenced from wikipedia and originally posted by Emoody26