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

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.

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.

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.

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.

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.

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.

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.

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.

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.