A sump is basically for putting in a skimmer and a method of filtration. This is the basic order of sumps, not saying that a sump needs to be like this or this applies for every sump, but it's just generally how a lot of people do it: filter sock->refugium(if they have one)->skimmer->internal pump or room to go out to an external pump. Mine is set up in a different order b/c my external pump is too strong and would create a hurricane w/in my refugium if I had it like that. Some people say refugiums are very beneficial, some say they cause higher nitrates. The main purpose of a refugium is to grow pods
and macroalgae that would otherwise be consumed in the DT. This is just info to help you make a decision if you wanted a refugium or not (don't know how much you know about that stuff). Wouldn't recommend bioballs, they can be a nitrate factory if not cleaned frequently and they are considered an old method of filtration aka there are newer, better ways. Just look up some styles of sumps and there's prob. an AA article on sumps/refugiums.