A sump is a tank that sits under the main display. The display overflows into the sump. The sump can be used in many different ways, one of which includes housing a refugium.
A refugium is any place that is protected from things in the display. It is usually used to grow macroalgae for nutrient export. In most setups it is in the sump, but may be a separate tank from the sump, a HOB
, or even in the display itself (such as in a small plastic container in the display).
Yes, the refugium can also be used to protect display inhabitants from certain things that may be kept in the refugium. I have a mantis in mine, I know other people who keep things like puffers, large hermits, etc. in the fuge. But I guess this still protects them (from the wrath of the angry aquarist when tank inhabitants are harmed).