about refugium

Why  have a refugium?

Also called fuges or refuge, a refugium is a dedicated space that can provide a number of benefits:

  • It can act as a living filter to restrict the growth of undesirable algae in the main tank by growing more desirable macro algae colonies that consume unwanted nutrients in the the main tank. (nutrient uptake)
  • It can be used to cultivate and grow plankton, amphipods, copepods, and other invertebrates protecting them from predation in the main tank and creating a natural live food source for main tank inhabitants. (live food source)
  • It can add to the total water capacity of your entire system, increasing stability.
  • It can help balance your tanks PH chemistry when established with macro algae on a reverse photo-period.
  • It can house a deep sand bed and help with nitrate export.
  • It can also serve as a suitable habitat for organisms you want but may not fit well in your main tank.
  • It can also provide for circulation, surface agitation, and oxygenation.

What do I need to consider?

How big should it be?

You can design it as part of your entire aquarium set up from the start or you can add one later. Including one in your initial plans can make the plumbing easier to execute. The refugium can be any size you want but generally the larger the refugium the more benefit you will see from it.  A suggestion of 20% of the size of the display tank is most often given1; with a higher percentage for smaller display tanks. If the refugium is too small it can be like having none at all. The refugium itself can be any shape that best fits your space and needs: It can be made from another tank, a container (plastic storage type) that holds water and be connected to your system, or a ready made product. Some of the more complex refugium designs also include areas or compartments for a filter bag, protein skimmer, bubble trap, return sump in addition to the actual refuge areas.

Care must always be taken not to set up a situation that could lead to overflow of either the sump, display tank, or refugium. Never try to set up a dual pump system where you have a pump putting water into and a pump taking water out. Such a setup makes it next to impossible to make sure both pumps are moving the same volume of water over a long period of time. It is a good idea to diagram out the set-up and water flow to discover and resolve potential problems before hooking it all up.

Where to put it?

There are several options for where to set up and run your refugium:

  • It can be below the display tank as a separate ‘tank’ or container attached to the system (sump type). Most commonly you have a gravity feed from the display tank into the refugium and a pump returning it to the main tank. You can also set it up on a secondary circulation feeding out of either the sump and returning back to the sump or out off the main tank and then back. Going this route you can also combine a refugium and sump into a single set up.
  • You can set up your refugium as a display tank next to or above your main display tank. A pump from either the sump or main tank feeds it and then a gravity drain returns the water back to the main tank. The below tank option seems to be general preference as it and associated plumbing and electrical can be hidden in a tank stand or other unseen area.
  • A hang on refugium is another choice, especially for systems that do not have a sump. This type is a container that hangs off a side (usually back) of the main display tank. A small pump sits inside the tank to feed water into the refugium and water drains by gravity back into the tank.
  • There are also in tank refugium. These are generally a small acrylic box with holes in the sides. They attach to the inside of a tank wall, the top sitting slightly above the tanks water level. A powerhead or similar pump moves the tank water through the box.
What can I put in it?

What goes in your refugium will depend on what you want it to do for you. It can be designed to create several different areas that perform different functions:

For nutrient uptake: The refugium is a place to grow macro-algae that will uptake ammonia, nitrate, phosphate, and heavy metals – food for these plants and in most cases unwanted products or toxic to the inhabitants in the display tank. Photosynthesis performed by the macro algae consume these unwanted items and gives off oxygen, increasing the overall amount of dissolved oxygen in the system and a benefit for the tanks animal inhabitants.

Taken a step further, if lit on a photo cycle opposite the main tank, called Reverse Daylight Photosynthesis (RDP™ ), it will help maintain a more stable pH by  removing the CO2 produced in the main tank during the night (nocturnal photo period) while maintaining a saturated Dissolved Oxygen (D.O.) level and so a more consistent pH level within the tank. Employing RDP is not necessary but has been proven beneficial by several experiments and lights can be run on the same cycle as the main tank. Some aquarists run the refugium lights 24/7.

Which macro-algae is a matter of personal preference, you will find common choices to be one or a combination of Chaetomorpha, Gracilaria, Halimeda, and Caulerpa.

There are also dark refugiums, an area for filtration that receives little to no light where a habitat is created for sponges and other non-photosynthetic filter feeders to grow. This type of filter system takes a significant time to mature, but does get better with age. These ‘cryptic zones’ can be created separately or by partitioning off areas of a sump or refugium, protecting it from any significant light source.

As a live food source area: To provide a place for copepods and amphipods to live means setting it up with their ideal environment. Foremost that means a place where they can reproduce without predation. Then, an environment they like; a nice mound of rubble or rock on a bare bottom or thin sand layer. It will not need to be lit. Pairs of invertebrates such as glass, mysid, and peppermint Lysmata sp. shrimp, and other amphipods / copepods reproducing on a regular basis will provide a near constant source of  micro fauna food for the fish, corals, and invertebrates in the main tank.

As a settling area: Providing a place for low, slower flow allows suspended solids such as uneaten foods, fish fecal matter, etc. to settle out where it can be periodically siphoned out or become food for detritus eating animals living within the refugium. Low flow, somewhere between 1 to 2 times the volume of refugium, per hour. For example, a 20 gallon refugium would have a flow of 20 to 40 gallons per hour through it. Too much flow can cause too much water movement and adversely affect fragile and small inhabitants.

You can also set it up with a Deep Sand Bed (DSB). It will need to be at least 20% of the display tank volume and able to hold a sand bed of 4-6″ (or deeper depending on the definition of deep sand bed you choose).


1Fenner and Calfo, Natural Marine Aquarium Volume I – Reef Invertebrates


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