Get rid of the bioballs. They are not suppose to be submerged, but need water trickling through them. Keep the charcoal out and be ready to clean the first sponge often.
Heck, I would put a filter sock at the drain line. Its better than the sponge IMO
. The second sponge would catch any plant debris from getting into the pump, but watch it carefully. If that sponge traps too many plant debris, then the flow will drastically be reduced and you get and your refugium area will overflow.
What kind of plants will you use? Are you going to use the refugium area for anything else?