I added a Diamond goby to the tank on 11-Jul-2020. She was totally rearranging the sand bed to her liking, living with the pistol shrimp, sifting sand, and seemed to be doing great. Last night she was "panting" heavily and this morning she was belly up and deader than a doornail. Parameters yesterday were: pH 7.8, ammonia and nitrite 0, nitrate 40. No injuries, lesions, or other signs of disease. I moved some frags yesterday and glued them in with 2 part epoxy (always used paleo glue in the past), changed the filter floss, and removed the hitchhiker sponge. Last night I fed pellet food for the fish (as usual) and broadcast feeding of reef roids. No real change to my normal routine. Any ideas?