Well SHOOT....my fish died.....Help If You Want.

[ColorfulRevenge]

Im not sure if it was a Bioload problem because all my water parameters checked out fine. My filtration was great, salinity is great, water movement was great as well as temp. The Damsel and Wrasse were fine and eating with good color. I got a couple of firefish gobys and had them in a well vented plastic container so they could acclimate to seeing their new home and tank mates. Well, I do believe I killed my tank. The damsel died the next morning (toughest punk went first) but the wrasse and the Fire gobys were hanging out being fish. The wrasses died before I went to work and the Gobys were still content observing. I come home after work...all dead...yay. Kinda XXXXX off, not about the money I spent, but not knowing what happened. Someone help me out here. Can seeing new fish cause them to die?