I bought something very similar to seasalted for three reasons: I got tired of fishing my wrasses out of the overflow, I was growing massive amounts of algae in there, and I was worried about a snail getting inside and stopping up my drain. You could probably DIY one much cheaper, but this is another option
http://www.limpitsreef.com/D-Shaped-Overflow-Cover.html
http://www.limpitsreef.com/D-Shaped-Overflow-Cover.html

