The words refugium and scrubber are often used interchangeably. In fact, a refugium is simply a refuge: a place for organisms to live in safety from the inhabitants of the aquarium, usually in a separate reservoir in the sump. An algae scrubber is a type of refugium that houses algae under a light. It is a good option to help reduce swings in pH. By lighting the refugium in a reverse cycle as the main display tank (i.e. at night), the plants will be photosynthesizing and absorbing CO2 from the water throughout the night when the CO2 would otherwise build up.