Defined as an area that has escaped ecological changes occurring elsewhere and/or a suitable habitat for relict species. In other words, a shelter or sanctuary tuned for a particular species and its environmental needs. Where aquarists are concerned, it means a secondary aquarium, connected to the main aquarium. One that acts as a holding vessel for the culturing of an alga and/or animal species for the purpose of improving the health of the main tank’s animals and water quality or holding those that would not easily or readily co-exist in the main system. Long sentence, but you get the point as has many aquarists these past few years because refugia use is growing and taking on many different forms.
As for refugia in general, some set-ups are solely for aesthetic purposes, where the aquarist wants a small add-on tank that can readily display a particular species that may not survive in his or her main aquarium. Then there are those that strictly serve as an additional ‘filtration’ system and are sometimes hidden from view, usually beneath the main aquarium. There are also those that are somewhere between those desires, and serve as added filtration and an area where micro-crustaceans can multiply and hopefully flow into the main tank, helping to somewhat act as an impromptu supply of live food. In fact, I had a refugium with a plenum that was used to grow sponges, as they did not need any light, liked the moderate current flow in the refugium, and also considerably helped to filter the water passing through this small add-on system. So there are many uses for refugia, probably only limited by one’s imagination!
Where the hobby is concerned, additional natural filtration is probably the basis for most marine refugia now in use. And one of the most talked about benefits is their ability to help reduce swings in pH when equipped with macroalgae and lit at opposite timeframes than the main system, which additionally helps to reduce the strain on overall system carbonate buffering (alkalinity). Of course, to what degree of usefulness is provided depends upon the size of the refugium and the condition of its macroalgae, and the size of the main tank and its bioload. Small one or two gallon hang-on-tank or very small sump-like refugia would probably provide little or no measurable water quality benefits to a very large show aquarium. Yet these small refugia can be very helpful on smaller aquaria, and wherever used, are quite interesting to watch as its developing processes take hold of its small environment.
In any size algal filtration refugium, its alga would need to remain healthy and regularly be harvested. If not cared for, it could easily disintegrate and add unwanted nutrients to the overall system’s bulk water. As for lighting such alga filtration units, I recommend the use of common daylight fluorescent lamps since algae prefer more of a red spectrum. Keep in mind common household lamps are higher in this wavelength since they are engineered to make colors look more like what ‘people’ think they should look like! On the other hand, reef aquarium lamps are usually higher in the blue spectrum, a favorite of corals, and are also far more expensive than common household lamps. Therefore, lamps high in blue do not need to be used to light a refugium where plant photosynthesis is the desired effect; nevertheless, they too will implement photosynthesis.
And where refugium use is utilized for exporting nutrients (algal filtration), certain species of algae should be given much consideration, as some species can provide far more benefits than others. For instance, Caulerpa mexicana and Caulerpa prolifera are the far better nutritional and ‘tasty’ choices for herbivorous fish, besides being an excellent way to export system nutrients. Therefore, two purposes can be fulfilled when these species are included in refugia design, i.e., removing nutrients from the system’s bulk water, and possibly serving as an excellent foodstuff for some fish in the main system. Yet, even though these algae serve a two-fold purpose, other forms of faster growing algae should also be considered, even though not as appetizing to many herbivores. For example, Caulerpa racemosa is extremely fast growing, and so is Chaetomorpha linium/other species in this genus, and possibly export unwanted nitrogen-laden compounds and phosphate from aquarium waters more quickly. And when these alga-type systems become well developed, other benefits begin to show up, e.g., increased micro-crustaceans growths, with copepods and amphipods becoming readily available for transfer to the main system as a live food source for both fish and corals.
Refugia can be very large or quite small, depending upon their purpose, available space, and the size of the system they are to be connected with. In fact, the one shown in an attached photo was a 10-foot long alga filtration system that was photographed in an aquarium shop in Northern California where it was connected to many different invertebrate tanks. As for much smaller refugia, they are sometimes part of an existing sump, oftentimes in the sump of a wet/dry filter system, as shown here in another photo. And some of these are not what could be called a ‘work of art,’ yet function quite nicely with practically no attention to their needs.
Besides the added filtration aspects, refugia are also seeing wider acceptance today as ‘niche’ systems that can be tuned to fit a particular species that would not have normally survived in a community-style aquarium system. Therefore, these interconnected systems of varying size and purpose can be designed to house animals and/or alga/plants that need an environment devoted to their requirements.
As to these more specialized refugia, one must give each environment desired some forethought, since the usually smaller refugium environment will share the same water quality as in the main system. In some cases where alga use is concerned, this may limit the species it contains, as some are element/compound dependant, such as the red macroalgae Gracilaria parvispora and Botryocladia uvaria. Without a doubt, these are very pretty forms of algae, yet are far from ideal as a nutrient exporter, or even that of a tasty nutritional foodstuff. Furthermore, they are Bromine dependant (pers. com./Martin Moe Jr.). And since bromine/bromides are quite often excluded/used in very low levels in the manufacture of artificial sea salts, a trace element additive would have to be regularly used if these red algae were the main reason for the refugium addition. And if a trace element solution containing bromides were used, then no ozone could be applied to that entire system, as ozone reacts with bromide ions and results in a free-radical known as hypobromous acid, a strong oxidizing chemical. Again, some forethought/planning needs to occur in refugium design prior to investing time and money.
As mentioned above, a non-lighted refugium where sponges and other low light organisms could be raised is also a good possibility. In fact, that’s exactly what I did in a past system where various sponges and other low light organisms, along with copious amounts of amphipods and copepods were raised in this refugium system. It became a focal point when visitors came, as some sponge and tubeworm growths were quite amazing!
Refugia can also make a great place to hold the bully, fish or invertebrate that may be in the main show aquarium while deciding what to do with it. And were flow between it and the main tank can easily be halted, refugia can also serve as a temporary quarantine holding area for a new comer to the main system. If the animal proves healthy, it can then be moved to the main tank and flow between both tanks resumed. If not healthy, it’s far easier to completely isolate/treat as needed a smaller system than a much larger main system, or simply remove the species needing a hospital tank.
Another reason some aquarists have for adding a refugium, one I disagree with, is they are of the opinion that by installing a refugium it will provide for a greater carrying capacity in the main system because it increases overall system water volume. In my opinion, one should realize that it’s not system water volume that is most important when it comes to carrying capacity; it’s system water ‘quality.’ Therefore, its filtration efficiency that determines the number, physical size, and type of inhabitants in any system (carrying capacity), not water volume (aquarium size).
Besides housing specific species of alga, fish, or invertebrates, refugium design often centers on the type of substrate and depth of some products used. Sometimes a mud-like product is used, as it helps foster growth of various forms of algae. Then again, and most commonly used, sand of different grain size and bed depths are employed. There are also bare-bottom refugia, as I’ve seen some that only had a large amount of unused live rock and old coral skeleton material in them and were simply serving as a storage area and at the same time, providing some additional filtration.
As for the equipment, refugia models come in all sizes and shapes including hang-on-tank units, and those having separate/dedicated areas for pump, skimmer, heater and various types of filter material placement. And manufacturers have seen this growing popularity and have specifically designed some additional equipment for the smaller systems, including nano aquariums, such as small-sized protein skimmers, calcium reactors, and phosphate reactors as added support for these aquaria.
In closing, whether it’s a sump area, partitioned areas in the main show aquarium, a commercially manufactured unit, or a connected small aquarium, the idea of a separate chamber, i.e., refugium, has merit. Stocking it with live sand, live rock, macroalgae, and some invertebrates, and maybe a small fish or two creates an interesting and often overall beneficial environment. Lighting that smaller environment with a photoperiod opposite that of the main show aquarium helps provide some overall system pH and alkalinity benefits. Yet, do its benefits outweigh overall additional system maintenance and costs? I think they do and if set up properly and maintained, they not only benefit the main system, they can be as interesting and fascinating as the main system! In fact, refugia use should not be limited to marine aquariums, as a freshwater plant/filtration/plenum refugium connected to a cichlid tank for example, can greatly limit high nitrate levels that frequently occur in said species tanks! Therefore, whether freshwater or marine the choices are many, which is so intriguing about this aspect of aquarium keeping!