Thank you all for the advice, I just dont have any other thoughts on how they could of died. A few clues besides the addition of a chiller. 1) I added a green carpet anemone attached to a small live rock piece 2) I added a few pound green mushroom rock 3) added a few lb. piece of tonga branch (all these additions along with chiller the night before the tragedy). A few days previous I notice some ich on a powder blue yellow tang, kole tang and Hippo tang (3 of 4 are no longer with us) I was treating them w/ garlic soaked food and cleaner shrimp. Remaining fish appear completey clean for a week or so.
I do test my own water with 2 seperate tests (instant ocean, and aquarium pharmaceuticals) all ammonia 0, nitrite 0, nitrate 0, ph 8.2, calcium around 450. All my corals look great (colt, mushrooms, polyps, octospawn etc.) just the 3 tangs died and a missing pseudochromis