New yellow watchman goby died

I got a YWG saturday, he did fine for a couple of hours, then jumped over the wall in my biocube into my sump.. He seemed to be fine Sunday, and today until about 2 hours ago, i noticed hermit crabs grabbing at him and he would just swim off a little.. He started trying to jump out again (ALOT) and then got really weak, getting blown around by pump and powerhead, and died shortly after..

Nitrate 0

Nitrite 0

Ammonia 0

PH 8.2



Help please!