I had this happen several times, most of the time I couldn't fish the goby out of my overflow due to space limitations in getting the net in there so I'd have to wait until he made it into the sump to fish him out (I have guards on all the sump pumps). I found there was really nothing I could do to keep him in the tank, sometimes he'd be in the tank for months, other times he'd make it into the overflow once a week. Eventually I just took him out and put him in my nano tank. He also had a sharknose goby chasing him constantly in the big tank so that could have been part of it. You'd think a 90gal would be large enough for two tiny gobies
