A [tag-self]refugium[/tag-self] is, as the name implies, a refuge where many tiny life forms can escape the hungry attentions of fish. The refugium is often a DSB ([tag-self]deep sand bed[/tag-self]) with macro algae planted. This way it serves two purposes – as said, it is a refuge and it also assists with the maintenance of high water quality.
Usually the refugium is in a separate tank either beneath or alongside the display tank. There are now ’hang-on’ refugiums available for those aquarists with a space problem. It is also possible to create a refugium within the display tank.
A refugium does not need to be large. If a DSB is to be created as well, then for the filtration to be fully beneficial to the display aquarium the DSB area needs to be somewhere near the guideline 2/3rds of the area of the display tank base. A DSB can be created in a smaller area of course, and it will still offer filtration benefits.
So what about a refugium in the display aquarium then? The aquarist is not going to create a DSB around 2/3rds of the display area! In this case it is obvious that the area is going to be considerably smaller, a ¼ or less. It doesn’t matter if the area is small as long as it is remembered that though there will be filtration it will be much less than a guideline-sized DSB.
The in-tank refugium does not have to be any particular shape. It can be a triangle formed by a dividing glass across a back corner. It can be one end of the display tank divided off. It can be a square made with the front set back from the front of the display aquarium and a further divider forming the other side. Or it can be a divider running the full length of the display aquarium, parallel to the back. The major consideration in these variations is access, it must be reasonably easy. Also, as this is the display tank it must be visually acceptable.
Seawater needs to flow in and out of the refugium. With it being in-tank this is very easily achieved. The flow rate through the refugium should be around three times the total system net gallonage per hour. To get seawater into the area, the glass divider, or one of the dividers, is made a little lower than the seawater level of the display aquarium. This creates a weir. Seawater flowing over this will assist in oxygen uptake and also the surface of the display tank will be kept clean of scum etc, again assisting in gas exchange. Seawater is returned to the display area by a pump. It is best if this seawater is returned to the opposite end of the tank. Depending on the gallons to be moved per hour, a simple powerhead could be sufficient.
Once the refugium area has been sectioned off, what goes in it? The same things that would go into an under-tank refugium. It is quite straightforward to construct a DSB in the area. [tag-self]Caulerpa[/tag-self] could also be used, but this depends on the actual area available. If it is impractical, it doesn’t matter. If Caulerpa or another suitable algae will go in then the display tank lighting could be used for it. A couple of small pieces of live rock could go in also(small as the sand shouldn’t be compacted).
Given some time the refugium will become just that. The aquarist will have another world to observe, with lots of tiny life forms inhabiting the area. In addition, if there is the space, items such as heaters can be removed from the display and hidden from view. Also there will be some additional filtration that can only be a plus for water quality.