Theoretically, a refugium can be set up like any kind of
aquarium system. Thus, if it is large enough, you could put virtually any
aquarium species in it. Of course, you might have to make some special,
additional accommodations; for example, if housing an eel, a refugium would
need a tight-fitting lid.
But, as their name implies, refugia serve one big purpose:
To separate and provide a refuge for flora and fauna that would not
survive (e.g. get eaten) in the main tank. By convention, this mainly includes,
but is not necessarily restricted to, pods and macro.
But, hey, if you have that entire “extra” tank running
“empty,” it’ll be too tempting not to put some fun stuff in it, right? In
addition to the pods and macro, there are a few items you could add.
Regarding flora, some aquarists have done cool things with
seagrasses and/or mangroves. There aren’t a lot of fishes that work well here…
most fish species are too big/active or eat all the pods/macro! However, there
are a handful of notable exceptions (Sargassum anglers, for example). There are
a lot more inverts that are suitable for refugia, so long as they are not
herbivorous. Things that can’t be kept in the main tank (i.e. not reef safe)
come to mind… mantis shrimp, chocolate chip stars, etc. Though this method of
filtration remains fairly arcane to date, some aquarists are experimenting with
sponge-dominated refugia (i.e. cryptic zone filtration methods) to strip excess
plankton and particulate organic matter from the water column.
It will be interesting to see what sorts of critters aquarists
attempt to keep in refugia--and what the results will be. To be sure, it is
best to err on the side of caution when thinking about introducing any sort of
animal to a fuge; remember always that there can be unintended consequences to
this, and that the priority of most refugia is to support the health of pods