good HOB refugium for 20 gallon nano?

A refugium (fuge) is used as a form of filtration for your display tank. Usually housing some type of substrate, macro algae and rock rubble. The Macro algae ( most commonly used is chaeto) acts as a nutrient and phosphate remover as this is what it absorbes or eats. Thus eliminating it from your display tank. Fuges are also used to grow/keep copods/amphipods which are a great part of the clean up crew and a wonderful source of fish food.

 

Great way to put it.

 

So almost like an extension of your tank used as a type of filtration and element neutralizer. Not necessary but another part of a tank to help everything run with the least amount of maintenance.... ???

 

Yes, simply put.