Squishy has passed on to the great big aquarium in the sky. I'm not too sure what happened. Squishy got into a very explorative and interactive phase, which was great. He was always running about in the tank, eagerly looking to see what treats I brought him that day. Then about a week ago he refused to come out of his cave, he would just stay in there. Another day and he was still in there. I dropped in a fiddler crab and he didn't go after it, which is when I knew something was wrong. I checked the water parameters, and everything looked good. Salinity was at 1.0255, ammonia and nitrites were at 0, though my nitrates were a little higher than I would have liked them. I did a 20% water change that night. The next day he still didn't eat. He wouldn't respond to my taps on the glass, as he used to in the past. The next morning I found him lying outside of his cave (he normally tucked himself up in the top, though it looked like he could no longer hold on). When I came home that night, he was in the back of the tank, dead. I did a quick look over his body. He had no marks of any kind. There were no other creatures in the tank with him except for a fiddler crab I had put in earlier as food (and he didn't get any marks from him). Nothing had changed with the tank, I didn't add or move anything, and I don't let people stick their hands in, so I don't believe there was any contamination. He was living in a 40 gallon that was connected to another 40 gallon (housing a few seahorses, hermits, and snails) and a 20 gallon sump. All other fish / creatures were doing well.