What is a Refugium?
According to The American Heritage Dictionary, a refugium is defined as follows:
An area that has escaped ecological changes occurring elsewhere and so provides a suitable habitat for relict species.
So what does this mean, and more specifically, what does it mean for a marine tank? To put it simply, a refugium is an area that allows certain organisms to survive in an area that is environmentally different than it's surrounding area.
Now let's look at what this means for our tanks. We tend to keep a variety of fish and inverts in our tanks, that require different environmental conditions in order to survive. Some are herbivorous, some are carnivorous, some are omnivorous. Some require intense light, while others prefer low light. Flow requirements are often different as well, with some organisms enjoying a brisk flow and some liking a more mellow water movement.
When we set up a refugium on our tank, we are looking to provide an area that varies from our display tank in one or more of these ways. In doing so, we are able to keep organisms that we otherwise wouldn't be able to.
What benefit does a refugium attached to my tank provide?
A refugium enables us to keep organisms that we wouldn't be able to maintain in our display tank. Let's take a look at how this can benefit us.
While there are many ways a refugium can be set up, I'm going to discuss two types for the purposes of this article. The first is a utilitarian refugium. A utilitarian refugium is set solely up to provide a benefit to the display tank, and is the more commonly seen type. A less common setup is a display refugium. While it can still provide benefit to the entire system, a display refugium is primarily used to house organisms that can not be kept in the main display.
Utilitarian refugiums are most commonly used to provide nutrient export for the main display, and often have the added benefit of housing a variety of creatures that can be a source of food for the animals in the display tank. This type of refugium is commonly set up with a slow to moderate flow, moderate lighting, and contains no predators. In order to provide nutrient export, a sand bed or mud bed is often established, and one or more species of macro algae are added. If the proper conditions are met, and the refugium is properly maintained, this can be an excellent method of removing excess nutrients from the system. This type of refugium will be the primary focus of the rest of this discussion.
The primary benefit of a display refugium is the ability to keep things that you couldn't otherwise maintain in your display tank. This could be a planted marine tank, a breeding ground, or even a species specific tank. This type of refugium should be set up to provide the conditions most suitable to whatever you decide to keep in the tank, so there are no strict guidelines on how to set up this type of refugium.
What size refugium is needed?
This is a very frequently asked question. The best advice I can give is to go big. A utilitarian refugium benefits the tank by providing nutrient export and adds a breeding ground for pods and other small creatures. Generally speaking, the larger the refugium, the more effective this will be. A display refugium can be any size, as long as it's well suited to the organisms it houses.
What kind of light do I need?
This question is a difficult question to answer. The size of the refugium, and the organisms you're planing on keeping in it are the primary factors in choosing a light. Chaetomorpha is a commonly kept macro algae, and is not overly demanding in its lighting requirements. Often a power compact bulb from the local hardware store will provide enough light, but in a deeper tank, you may need to consider a better source of light. Research what you plan to keep, and light the tank appropriately.
How long should my lights be on?
Once again, this depends on what you choose to keep. I generally don't advise a 24 hour photoperiod. A commonly accepted method is to light your refugium when the tank lights are off. This aids in easing the pH swing in your tank.
What kind of flow do I need?
The most common problem I see with refugiums is improper flow. Many people set up their refugium in a chamber within their sump, and depend on their return pump to provide flow. This is almost never ideal, and I strongly discourage this type of setup in most cases.
There are two types of flow to consider when setting up a refugium. The first is the water flowing through the refugium and back into the system. For efficient nutrient export, a fair amount of contact time is needed. This means that a slow, steady flow through the refugium, will generally provide the most effective nutrient export. The second type of flow that needs to be considered is the flow within the refugium. Many types of macro algae require moderate to high flow. This can be achieved by placing a powerhead within the refugium if necessary.
A display refugium's flow should be dictated by what you decide to keep in it.
What should I put in a refugium?
There is a great deal of debate over the best organisms to keep in a refugium. In a utilitarian refugium, a sand bed is not required, and not advisable unless it's a minimum of 4" deep. If you choose to keep a sand bed, you'll want to ensure that it is disturbed as little as possible. While a mud bed can be a great breeding ground for pods, appropriate mud isn't cheap, and generally a sand bed will suffice. There are many types of macro algae that will provide excellent nutrient export. The two most commonly used are chaetomorpha and caulerpa. The latter has a reputation for "going sexual", but if you read and understand the risk involved in this, you can make the decision on which to use.
What should I avoid putting in a refugium?
There are many things that you'll want to avoid in a utilitarian refugium. The primary thing to avoid is predators. You want to ensure that nothing in the refugium is going to disturb the algae or eat pods. Snails should be placed in a refugium with caution. While most won't cause a problem, a properly running refugium will be a very low nutrient environment, and it's unlikely that they will have enough food to survive on. There are some types of algae you should avoid as well. Calcareous algae should be avoided in a utilitarian refugium. It will pull vital elements such as calcium from the tank and provide little benefit. Mangroves should also be avoided. In order to provide nutrient export, their leaves should be misted daily with freshwater. In addition, over a long period of time, mangroves can develop extensive root structures, which can do damage to the tank they're kept in.
Do I need a Refugium?
If you have the space and the means to set up a proper refugium, it can be a great asset to a reef tank, providing nutrient export, a breeding ground for pods and other micro organisms, and in the case of a display refugium, a beautiful secondary environment. If you do not have the space or the means to do this, I would suggest not bothering with one at all. The benefit of an improperly set up refugium is minimal, and the space is better used by a fluidized reactor or larger skimmer.
Well, that's it for now. Hope this helps answer some questions!