I have had a 55 gallon saltwater tank running since December, live rock put in in January, a yellow tail damsel two weeks later and now after many tests completely satisfied with my levels I finally bought a yellow tang yesterday. I was a little nervous about how the damsel and the tang would do together but for the four hours that I watched them, absolutely no problems, the tang was a little skiddish, hiding in the rockwork, but other than that, seemed ok. Well this morning the tang was dead. I have tried to do everything by the book with this tank, protein skimmer, wet/dry filter, perfect circulation, I gave the tank cycling time and before I left to purchase the tang I tested the water and ammonia and nitrite were zero, nitrate was almost zero, specific gravity 1.022 and temp 78. Could someone please tell me what could have caused this fish to die.