A refugium is, quite simply, a place where organisms on the bottom of the food chain can grow and thrive without predation.
Common implementation of refugiums are in a sump. However places like the weir, in tank "cages" and hang on refugium types are all options for this.
The main objective is to have critters like 'pods, polychaetes, and other microfauna reproducing. Most of these critters are not only detrivores (wonderful cleanup crew) but their offspring and even themselves find their way back into the tank and become food for fish and coral. Other beneficial organisms include algae, but this is mainly directed at the Macro algae refugium
If implementing a refugium, it is favourable that there is no pumps between it and the main tank. For most systems this in unavoidable, especially a sump implementation. The next best thing would be to use air lift pumps. However even using normal pumps, a decent percentage manage to survive the shear forces in normal impeller return pumps and make it to the main tank
Macro Algae Refugium
When many people talk about implementing a refugium, they are usually referring to a Macro algae refugium. The difference between a normal refugium and a macro algae refugium is that a light source is introduced, along with some fast growing macro algae such as Chaetomorpha.
The growing of macro algae in a refugium provides the ability to export nutrients, by the removal of excess algae growth. The growth of algae absorbs the nutrients and by harvesting the algae the system is rid of the nutrients. Many also use this excess algae to feed herbivorous fish like tangs and rabbitfish.
An algae turf scrubber works on a similar principle, however turf algae grows faster than macro algae.
Possible cons are that algae can release calcification inhibitive toxins into the water. It is doubtful, however, that this has any additional significant impact on coral growth in an aquarium.
Simple desk lamp being used as the lighting for a refugium.
Basic refugium located on the left of this sump
Should my refugium lighting be opposite to my main tanks lights?
It can be, the reason for this is to help off-set the changes in pH that occur in the main tank during the day/night cycle....
When the Coral and Algae in the main tank are exposed to light, oxygen is produced as a byproduct of the photosynthesis process and carbon dioxide is consumed, this removal of carbon dioxide from the water causes the pH to rise. So when coral and algae are not exposed to light the production of oxygen is reduced and the production of carbon dioxide is increased, which leads to a decrease of pH during the periods of darkness. These fluctuations can be eliminated to a great extent by lighting a refugium in an opposite cycle to the main tanks, minimising the pH fluctuations and maintaining the amount of dissolved oxygen constant in the water.