tank crashing need help

my 120 reef tank is crashing and dont know why, test are all ok the only thing i can think of is my co2 i just added a new bottle and acouple days later i lost my yellow tang really both of them, also my naso tang and a bicolor angle plus my reef life ie corals are not looking to good, is it possable to get bad co2 everything was looking really health then all the sudden not so good, i shut off the co2, well any help would be great thanks
 
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