Yellow Watchman Goby just died

[Koresample]

I got him about 2 weeks ago from J&L. He acclimated fine and i never saw anything pester him. When i fed the tank, he was always there gobbling up mysis. Same thing this morning, then at lunch i went to look at the tank and saw him upside down on the bottom and he would literally convulse every 20 seconds or so and his mouth was wide open. I don't have a QT so i just had to remove him....anyone have any ideas as to why this could be? I have two false perculas and a mandarin, plus a coral banded shrimp and clean-up crew, they all look fine. I just tested my water last night, An, No2, NO3 all zero, ph 7.8, sg 1025.