I have a piece of filter foam sitting at the top of the overflow, mostly preventing fish from getting in. Since I have had this set up, I have pulled 4 neons and still have a cherry red in my sump. I am thinking of putting some lights on the sump and adding some moss to make the sump into a grow out refugium for the shrimp.I would not do tetras in a big tank I had some zebra denios and they kept getting stuck in the overflow and would find them flopping around the filterpad in my sump but if you must gutter guard helps cover the overflow