I purchased a 135 gallon reef ready tank with two glass overflow boxes. When I removed the black acrylic overflow covers I saw that I have two holes (one larger one smaller) in each overflow boxes. I was planning on using the overflow box on the left for the intake (going to go with a Durso standpipe) and the overflow box on the right for the return. So, what are the other holes for?

Next question - am I supposed to silicone the overflow covers to the glass? If I don't hold them up against the glass there is a gap at the top between the glass and the cover.