There's not much alive in my overflow box since I cleaned it out a couple of weeks ago to rescue a wayward Anthia. However, when I first looked in the overflow, aside of 2" of buildup in the bottom of each box over the last two years, I was surprised to see large colonies of feather duster worms. These were not the ones you buy at the store nor were they the micro versions that live on live rock. They were about 4" long, but only about 1/8" in diameter. Very interesting. I'm sure there are also all other kinds of things in the overflows, but I painted my back glass so I can't see into my boxes like you can while the tank is running. Finally, I made some plastic covers for my boxes to make sure no fish get back there again, but I think about once or twice a year I will start vacuuming out these boxes during water changes. The smell of sulfides was strong! :)