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A refugium, like the name suggests, is a refuge for plant and animal species that may prefer a less hectic environment than the main tank. In addition, populating a refugium with the right critters can be very beneficial to biological filtration, and generally maintaining a clean, healthy main tank.
The Multipurpose Refugium
Refugiums vary in size and purpose. Generally one-tenth the size of the main tank, they can be populated with oxygen-producing plants or toxic nitrate-absorbing organisms to assist in biological filtration of the main tank. Or, they can function as breeding tanks for organisms that will later be moved to the main tank.
There's quite a lot of variety among saltwater snail species. Some peacefully feed on algae, while others are much more predatory. An aquarist can help determine what is best for each tank, but if the refugium is intended as a filtration system, choosing the right snails or other organisms is critical to ensure the refugium operates at full potential and that algae and plant material are sufficient to remove toxic debris and oxygenate the water being cycled through from the main tank.
An Ideal Refugium
A proper filtering refugium will contain a mix of high-, medium- and low-nutrient uptake plants. As fluctuations in your display tank occur, these various plants can respond as needed to establish a healthy tank equilibrium including appropriate oxygen levels and the removal of toxic byproducts from fish waste. Allowing snails to destroy these plants may negatively affect the main tank water quality.
To Snail or Not to Snail
If using the refugium for aesthetic purposes or to raise juvenile fish, snails or other organisms, there is much more flexibility as to what features it can have and it provides a smaller, quieter, perhaps less stressful environment prior to transfer into the main tank. Once in the main tank, snails can either function to remove algae or as fish food. But if using the refugium to cycle, filter and maintain balance in a main saltwater tank, algae and plant-eating organisms, such as snails, should be limited.
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