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What is a Hang On Tank Refugium for a Reef or Saltwater Aquarium?

Updated on April 20, 2016
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Kylyssa Shay has successfully kept aquatic creatures for decades. Her fish care advice has been published in Aquarium Fish Magazine.

A lawnmower blenny peers over an elegance coral in my reef aquarium.
A lawnmower blenny peers over an elegance coral in my reef aquarium. | Source

What is a Hang-On-Tank Refugium and What Benefits Does It Provide?

A refugium is a device used with home reef aquaria and saltwater fish tanks which provides a safe haven or refuge for plants, animals, and filtration materials. It keeps them away from the inhabitants of the display reef aquarium and, in the case of external refugia, provides extra water volume to dilute wastes. It provides a discrete location for filters, heaters, and skimmers that may detract from the appearance of the display tank.

It also provides easy access to the equipment like heaters, filters, protein skimmers, and pumps inside it. It's also nice to not need to keep an eye on large carnivorous fish that could curiously bite off a finger during tank maintenance.

Many saltwater hobbyists use refugia that are separate fish tanks, often located under the display tank. A hang on tank refugium hangs on the outside of the aquarium, usually in the back, and provides the same benefits.

Advantages of a Backpack Style Unit

One of the easiest types of lighted sump to use is the hang on tank or H.O.T. style. This type of filter is also sometimes called a hang-on-back or H.O.B. They install simply, usually just by hanging on the back of your home reef aquarium and plugging the pump into an outlet with a ground fault interrupter to prevent accidental electrocution. While not providing as much extra water volume as an average under-the-tank refugium or sump, the hang-on-tank refugium is ideal for any reef aquarium without enough space for filtration beside or beneath it. I would recommend a backpack style refugium to all reef aquarium or saltwater aquarium beginners and to anyone looking to buy a device to increase water volume which is extremely easy to set up and maintain.

Macro algae kept in a refugium can help break down fish wastes and produce food and oxygen while sheltering beneficial detrivores.

Large Hang-On-Tank Refugium Choices - HOT refugia

While these devices do not provide as much water volume as traditional sumps located under or beside the tank, they can take advantage of natural light and tank light. They can be lit with dedicated lighting to grow macro-algae for waste nutrient export. The extreme simplicity of their set-up makes them a great choice for beginners.

They do not require the complicated plumbing that full-sized models located under or beside a tank do. While they don't provide as much extra water volume, they still provide a refuge for new coral frags, microfauna, and macroalgae.

CPR AquaFuge 2 Hang-on Refugium with Protein Skimmer, Large
CPR AquaFuge 2 Hang-on Refugium with Protein Skimmer, Large

The included protein skimmer is a real bonus because it fits this model perfectly.


A refugium provides a safe haven for young, small, or injured fish and invertebrates and for coral frags until they are ready for life in your display aquarium.

When I used a medium H.O.T. holding a few pounds of live sand and chaetomorpha macroalgae on a twenty-nine gallon tank stocked with a green spotted pufferfish and four damsel fish, the water quality was pristine even when I was a bit lazy with water changes. I don't recommend skimping on the water changes but the even a small amount of added volume holding live sand and macroalgae can give you a little leeway.

CPR AquaFuge2 Hang-On Refugium - Medium
CPR AquaFuge2 Hang-On Refugium - Medium

This medium-sized filter provides ample room for heaters, thermometers, and probes.


Medium H.O.B. Refugium

These refugia are about as close to plug-and-play as any reef aquarium equipment gets. This size would be a good addition to any small to medium-sized reef tank or fish-only aquarium.

Protein skimmers, heaters, thermometer probes, and filters can be tucked away in your refugium to provide a tidier appearance for your display tank.

Be Warned - Yours Won't Look That Pretty

I'm sure you probably already realize this but the pretty pictures used in ads are not really representative of how your refugium will look in real life. Mine is crusted in coraline algae and stuffed full of an amorphous blob of macro-algae. Fortunately, no one ever really looks at it because it hangs behind the tank. But it does fulfill its function of providing extra water volume, a place to grow plants for nutrient export that my tangs can't reach, and a place to put equipment where my favorite pufferfish can't chomp it.

Small Hang-On Back Refugiums

While I think you should get the biggest unit your system can support, even small refugia give you a safe space for macroalgae to grow when you have plant eaters and a nice place to hide things like glass heaters from fish that may attack them such as puffers and triggerfish.

photo by Kylyssa Shay
photo by Kylyssa Shay

A refugium provides a place for macro algae, copepods, and amphipods to grow, safe from the predations of fish and invertebrates.

photo of Diodon Holocanthus by Kylyssa Shay
photo of Diodon Holocanthus by Kylyssa Shay

The biggest advantage to having a volume of water outside my display tank as far as I'm concerned, is the ability to keep my porcupine puffer from crunching glass heaters and thermometers or biting through equipment cords.

I use cheatomorpha macro-algae for nutrient export and it grows densely and well under a simple compact fluorescent light fixture in my hang on unit.

CPR AquaFuge2 Hang-On Refugium - Large
CPR AquaFuge2 Hang-On Refugium - Large

I'd recommend this size of refugium for tanks 29 gallons and up.


AquaFuge Hang-On-Tank Large Refugium

This large H.O.T. refugium is similar in design to the one I use on my 95 gallon saltwater aquarium. It provides 4.7 gallons of extra water capacity and fits nicely between the wall and the back of my display tank. It has proven very useful in keeping the thermometer probe out of my pufferfish's reach as well as providing extra filtration and space for macroalgae to grow.

Since my tank doesn't have a big cabinet underneath in which to hide a sump, a hang on unit has proven to be the best way to add more water volume and grow macroalgae my tangs can't get into and devour.

This refugium measures 25 inches long, 4.5 inches deep by 12 inches tall.

© 2011 Kylyssa Shay

What Type of Hang On Tank Refugium Do You Have/Want? - And what type and size of aquarium do you have or plan to put a one on?

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