I once had a baby clown go into my overflow and down the pipe to the sump, I found him swimming around and I got him right back into the tank just to find him back in the overflow the next day. It's very annoying to keep catching fish in the over flow. I have covered my overflow but still found a neon goby in it after being covered. Tried my best to get him out but couldn't, unfortunately I think I lost him after 3 days of being in there