i used to have a 15G biocube and we kept losing gobies, literally had at least 3 disappear consecutively in about 2 weeks. well one day i was cleaning out the sump in the back and low and behold theirs 3 gobies living back there for 2 weeks! poor things look like scrawny death. i got them all out and put them back in the display and then had 4 gobies, so i blocked the hole off. LOL.