Several years ago, Freshwater and Marine Aquarium Magazine (FAMA) and other aquarium periodicals referenced the idea of a refugium as part of a saltwater aquarium. It resulted in a flurry of "how-to" and "why for" articles that occupied many pages in aquarium magazines and continues to be the subject of new inquiries.
Essentially, a refugium is nothing more than a refuge from predation. Sessile invertebrates, such as corals and anemones, and other delicate species need a place to call their own, and the advent of the refugium was just the ticket. Isolated but connected to the main display tank (see diagram), the refugium allows for common water filtration while at the same time keeping more aggressive fish from impacting the delicate creatures.
What follows is a diagram showing a simplified refugium setup. Notice how the water is delivered to the unit and how it drains back into the display tank and/or sump. Notice that a true refugium is not a reverse photosynthesis device—those are another animal altogether. The true refugium is meant to be seen, as is the main show tank. Reverse photosynthesis arrangements are meant to be installed in a sump beneath the display tank and are a means of primary biological filtration. This is not so of the refugium.
The focus here is to provide a "quiet" zone for seahorses, peppermint shrimps, copepods and amphipods, and other denizens that would otherwise not last long at all in a community aquarium. While benefiting from the improved water quality that most reef systems provide, this isolation allows for spawning of shrimps or other animals, as well as providing a nice sand bed for beneficial worms and other sifters.
When contemplating installation of a refugium, several factors should be considered. First, where are you going to set it up or place it? Most agree that placing the refugium as close to the main tank as possible is the way to go. Many aquarists have simply set a 10-gallon aquarium on a table next to its big brother. This works, but requires a water return pump unless draining back into a sump or main tank. So where does the water supply come from, and where does it go?
Plumbing the Refugium
Water is pumped into the refugium using an aquarium powerhead pump, or other circulation from the filter pump. Then the water returns from the refugium into the main aquarium, or the filter sump, by gravity or by siphon action.
Time to make a point: The water entering the refugium should come from below the main tank's water surface—not from the overflow—if so equipped. Why? Surface water is skimmed from the main tank into the overflow. This surface water is laden with oils and other dissolved organics. Why would you want to dump this into your refugium? It's better to place a small powerhead inside and just below the water surface of the display tank to provide oxygenated water into the refugium. Better yet is to tap off the return line from your protein skimmer. Highly stripped and highly oxygenated water from the skimmer provides as near ideal water as you can expect.
Draining the refugium back to the sump allows gravity to do the labor for you. If your system is sump-free, then mounting your refugium above the main tank waterline or behind the tank may be desirable.
What to Place Inside the Refugium
Now you know what a refugium is and how it should be connected to the balance of your existing system. You've seen why it's important to consider the right plumbing and to plan accordingly. Now learn what to place inside, preparatory to introducing your animals.
Live Sand and Rock
Consider using live sand and some live rock. Since primary filtration is not an issue, the aesthetics of your creation take priority. It's recommended to use a sand bed at least 1 1/2 inches deep, two inches being better. Ten-gallon aquariums are only 12 inches deep so you will have to judge the sand volume for yourself. Think about establishing an area for natural denitrification. This aids both tanks being linked to a common water source and a deeper sand bed makes this possible. You can use live aragonite sand; however, there are many sources for sand as well as live rock. Place a few pieces of rock rubble from your main tank into the refugium. Once again, if you're using the main tank's filtration, biological considerations are not necessary.
What About Lighting?
There are a few ideas that allow light to be redirected over the refugium, but they are usually stop-gap measures. It's better to have a proprietary lighting system designed strictly for the refugium. A 6500K and a mono-phosphor 7100K blue provide nice lighting for a tank this size. One that's recommended is the Energy Savers Unlimited (ESU) Coralife bulbs or similar OSRAM brand of lighting. These are ideal units that feature twin OSRAM fluorescent bulbs in a neat aluminum reflector. OSRAM lights will give the intensity needed for your refugium inhabitants. Being just nine watts each, they radiate at a 75 watt, incandescent level. That's equal to 150 watts of incandescent light in a 10-gallon tank. That's plenty of beneficial illumination. You can use an egg crate cut to the inside dimensions of the tank frame and simply place the reflector on this to create an instant light hood. The reflector keeps stray light out of your eyes and keeps the rest of the room comfortable.