marina hang on breeder box outflow question

I use the boxes for their design purpose of rearing juvenile fish. So that depending on the size of the fish in the box, I have to block off the small overflow in some manor. I usually use small blocks of small celled filter foam like what's used in HOBs. I keep a bucnh on hand and just remove, replace and squeeze out the old one that's clogged. The center overflow is designed as a safety mechanism in case the smaller one become clogged or obstructed in some fashion. So that the choice of material and how often you clean what is in the smaller overflow will determine how much, the center overflow sees water, if at all.