What Is A Refugium
Why people use refugiums with their aquariums? Different types of refugium - and more.
The Refugium is a mystery to most beginner fish tank owners. Most hobbyists just starting out are so overwhelmed with the basics of fish-keeping, that they don't even realize that they exist. A refugium is a wonderful addition to the aquarium, however, and one you definitely need to learn about. In this article, we will discuss why people use refugiums with their aquariums, go over the different types of refugium set-ups that people use, and provide general tips and guidelines with using a refugium with your tank. We'll end the article with a list of our favorite online sources for refugiums.
What is a Refugium?
Technically speaking, a refugium is a 'refuge" in the sense that while it shares the water of the main tank, it is somehow set off from the tank. A refugium is generally used to provide delicate species with safety, protecting them from the larger fish in the main tank. Refugiums are often used to grow copepods as food for their aquarium fish, or to raise delicate species of fish or coral. A refugium can also be sued to filter the water from the main tank, acting as a sort of overflow tank or as a way to make water changes easier and safer for the tank.

Image by Flickr user Scott361
Why use Refugiums?

Given the purpose of a refugium, you may be wondering -why not just keep a separate tank? There are many cases in which having a refugium connected to your main tank may be beneficial. Some of these reasons may include:

  • With additional lighting, can be used to grow algae to harvest. Growing algae in a sump/refugium also helps absorb waste that would otherwise pollute the main tank.
  • Protect small organisms so that they can be self-replenishing. This works especially when you want to grow "feeder" organisms to a certain size so that they can reproduce before the entire population is eaten. Popular "feeder" organisms include feeder fish, amphipods, and copepods. For example: in a reef tank, a large enough refugium can provide a safe haven for copepods (where they do not have to worry about being eaten and can breed). When the refugium is "stirred" (i.e., when you trim the macro-algae in the refugium), copepods will enter the water stream and makes their way back into the main tank, providing a regular source of sustenance for the larger organisms.
  • When used as a sump pump, plays a crucial part of water flow for reef tanks.
  • Increases overall water volume of aquarium, making it more stable. For example a 60 gallon tank with a 10 gallon refugium is actually a 70 gallon system; the extra volume makes the tank water less susceptible to variances in temperature and other disturbances.
  • Separate fish that are delicate or prone to bullying (for example, seahorses) on their own. Some owners even place new fish in a refugium to help them acclimatize in a quieter environment (away from the inhabitants of the main tank). Helps fry develop in a safe habitat as they grow, while keeping them accustomed to the water/conditions of the main tank.

Main Types of Refugiums

When it comes to setting up a refugium for your tank, there are a few different options to choose from. Refugiums can be categorized into the following three types: sump based, in-tank, and hang-on. Each type of designed for a specific purpose and is best for certain types of tanks. Before you select a refugium for your tank, take the time to learn the basics about each option below:
Sump Based

With a sump based refugium, the refugium is a second tank which you place below your real tank. This "work" tank is usually kept under wraps and handles your water pump action for reef tanks as well as manages extra filtration.

  • Pros - Can keep your skinners in here; run a reverse photosynthesis set-up for your algae; not worry about how it looks; costs the least out of the different types of refugiums available if you go low-tech (you're basically dealing with a regular tank here, not a specialized piece of equipment).
  • Cons - You might forget to check your little fry as often as you should if you are keeping them down here; Costs the most if you go with a high-tech refugium sump pump option
  • Cost - Depends on how you approach the refugium idea. If you go with a low tech approach and use a plain tank, your price depends on your materials and size. However, if you go high-tech with a refugium that incorporates the sump pump and a protein skimmer, the resulting cost can be in the $350 - $600 range. The extra cost is for a "ready to run" tank with all the parts and plumbing ready to go.


The In-Tank Refugium is the simplest solution for refugium. You basically place a small container into your tank that lets the water in and out but keeps the critters in and your bigger fish out.

  • Pros - Doesn't take up any more space than what you already have; you can enjoy seeing the little fry or other little critters growing in here; cheaper than a hang-on type refugiums
  • Cons - Doesn't add water volume to your tank. Looks unnatural. Can't run separate lighting schedules for the main tank and the refugium.
  • Cost - Price range is $35 to $50 depending on how big of a refugium you buy.


Hang-On Tanks work just like their name; they are separate containers that hang-on to the back (typically) of the regular tank. The use an extra pump to move water in and out of the refugium.

  • Pros - With the Hang-On refugium, you still share the water between your tank and your refugium. Many tanks and stands aren't fitted with a sump, so this may be your only available out-of-tank refugium option.
  • Cons - Can't be too big or heavy if they're going to hang on to your tank; may not work with how you have your tank set-up relative to the wall (you'll need some space behind your tank to fiddle with the refugium). Can also be hard to clean water spills behind the tank. More expensive option than an in-tank option.
  • Cost - Price range is $75 to $150 depending on how big of a hang-on refugium you pick up.

Refugium Tips and Guidelines
Here are some general tips and guidelines when incorporating refugiums into your aquarium set-up.
  • There must be sufficient water flow through the refugium. The actual flow rate (how many times the water cycles through the tank) depends on what you're keeping in the refugium. A basic rule of thumb is to change the refugium water volume in one hour (i.e. for a 20 gallon refugium, the water flow should be 20 gallons per hour (GPH).
  • You can put your skimmer(s) (if you're using them) into your refugium if you're using a sump pump refugium. This can help free up valuable space underneath your tank for a calcium reactor and other equipment.
  • Consider how you want everything connected if you have multiple refugium tanks, sump pumps, filter tanks, etc. tied in and out of your main tank.
  • With refugiums as is with tanks in general, bigger is almost always better. Even if you think you won't need it now, get as large a refugium as you can afford and fit into your set-up. Some experts suggest sizing a refugium so that it is 20% the size of the main tank.
  • Heads up: People often refer to the refugium as a "fuge" or you might see 'fuge in print.

Inhabitants for a Refugium

As you may already know, a refugium can be used for a variety of different purposes in the aquarium. One of the most common uses, however, is to raise delicate species or sources of food for the fish in your tank. Some individuals like to use a marine refugium to cultivate certain types of algae to serve as food for marine fish – marine refugiums may also contain live rock and/or live sand. Freshwater refugiums are more commonly used to raise small organisms like brine shrimp, Mysis shrimp, copepods and isopods. A tropical freshwater refugium is likely to contain various species of live plant as well as certain scavengers like shrimp and snails to control waste accumulation. By keeping certain tank inhabitants in a refugium rather than in the main tank, you can offer them more customized care. For particularly small inhabitants like brine shrimp and copepods, it is beneficial to separate them so they do not get eaten by the fish in your tank until they are of the proper size. Keeping a separate refugium also ensures that the inhabitants do not make a mess that affects the rest of the tank.


Now that you know the basics about refugiums for the home aquarium, you may be ready to decide whether one is right for your tank. A refugium can be used in virtually any type of aquarium whether it be saltwater, freshwater or brackish – it all depends on what you want to use it for. As you already know, refugiums can be used to house sensitive tank inhabitants or to cultivate certain species separately from the main tank. More simply, you can also use the refugium as an extra source of filtration for your tank. If you decide that a refugium would be beneficial in your tank, take the time to think about which type is best for your tank and do some research to figure out how to stock it most effectively for the purpose you have in mind.

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