Argh... I am starting to hate the "assistants" at LFS
:P - Okay a refugium is a "sump" (in this case a tank) that houses mainly macroalgaes, sand bed (your prefereance), and live rock (again, your preferance if you want it there). I think what you LFS
guy meant by hospital tank is not for treating illness, but to keep invertabrates or certain fish safe from predators like certain shrimp, copepodes, or even other fish being overly harassed by others. Like a sump you can also house equipment like a protein skimmer, a heater, etc.
When I think of sump, I think of bio balls which is what I believe most sumps are. The benefits of a refugium, in my opinion, far outweigh the benefits of a regular sump. A refugium keeps macroalgaes that constantly do their own feeding (aka filtering). Liek other plants they use photosynthesus to make their own food.. using carbon dioxide. Whats good about this is that any unused carbon in the water eventually gets dissoved forming carbonic acid. Carbonic acid makes the water drop in pH (screws with the alk
of the water, which mean the buffering of pH gets messed with). On top of the pH ordeal, as I mentioned before you have other living organisms that can live in peace and not in predation. Many people grow shrimps or other food for the main tank inhabitants.. plants as well. Thats a short summary for you of what a refugium is.