A refugium in the world of science is a habitat that hosts an isolated population of a specific species as opposed to a variety of species. When speaking in terms of an aquarium, it is a place where organisms that are not found elsewhere in the tank survive away from the other inhabitants of the tank. Refugiums use the water system from the display tank, making them an appendage to the main system. Organisms inside the refugium like, feeder fish, specific bacteria, macroalgae, etc. are often kept to create natural filtration. Refugiums have many biological benefits and give your tank an additional layer of control.
Many will choose to place live rock and sand inside of their refugiums to create biological filtration. Macroalgae is another common addition to a refugium as it filters and can also be used as a nearly endless supply of food for the herbivores in your tank. Refugiums are a great place to raise feeder fish for your main inhabitants as they can be given the space and time to grow to substantial sizes that they wouldn’t be able to achieve in your main tank. Refugiums are also useful for fragile inhabitants that require a slower flow of water that may not be achievable in the main tank, such as Harpacticoid copepods, Amphipods, Fireworms, and the list goes on.
A refugium requires you to have a sump to house your refugium, or a container that prevents your inhabitants from accessing it. There are many pros and cons to each of the different setups, it all depends on what you are looking to get out of your tank. A sump refugium is the most common style. These types of refugiums are often called the “work tank” as it houses all of the working equipment for your tank. Sump refugiums are often the roomiest and can be extremely specialized, giving you the option to control a separate lighting schedule that runs opposite of your display tank. The downside to having a sump refugium is that it is more costly than the other options, and requires you to either modify your current tank or purchase a reef-ready tank.
Another option is an In-Tank Refugium, these are often more popular due to their simplicity and can be installed in any existing tank. As their name implies, these are containers placed inside the tank which hold their contents and tank water, while keeping out your display tank inhabitants. These refugiums are also often very budget friendly and can also be easily DIY’d if you are handy! The main downside to the in-tank setup is that they can look unnatural and can ruin the aesthetic of your tank. The in-tank setups also rely on the lighting of your main tank, versus separate customized lighting specifically for a refugium.
Lastly you can choose a Hang-On Refugium, these are separate containers that are attached to the back of the display tank. These refugiums often use an extra pump to flow water in and out. Hang ons are perfect for situations where a sump refugium or in-tank refugium is not convenient or possible. The biggest downside to a hang-on is that they can be more expensive and require you to make room for it behind your tank. Without proper space they can be inconvenient to clean and maintain.
Refugiums are an effective system that can improve virtually any aquarium setup. The ability to have a natural filtration device, as well as the added housing for cultivating species separate from your main tank gives you many more options for being a successful tank keeper. Decide what will be best for you, and be sure to do your research!