After a routine water change, I went to pick my girlfriend up from campus, when we got back, our rainbow shark was dead but his tank mate the opaline gourami was alive. I quarantined the gourami and went to petsmart to get the water tested, but all the tests were fine. The ammonia was a small bit higher than recommended, but it shouldn't have been enough to kill the fish. The tank has been running for approximately 2 and a half months and the only fish that has died was a blue gourami that my opaline and rainbow killed and ate. I treated the water before adding it to the tank, which from a biological and chemical standpoint seem stable at least from tests. Does anybody have any idea what could have killed my rainbow in such a short period of time? The water temperature was also about the same as the water added so it shouldn't have been shock. Any theories would be appreciated as if its something i did and can avoid in the future i'd like to know about it.