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If you have answers, please help by responding to the unanswered posts.
A question like the one you asked is difficult to answer without knowing all about your tank. My answer to you without knowing your tank would be for you to find out what is either wrong with your tank, your acclimation methods, or your fish compatability. Clownfish are pretty darn hardy.
Maybe your clown got the clown fish disease, do they have any symtoms before they died? Like white grey film on their skin? BTW what clown fish do you keep? You keep 3 of them in a tank, don't tell me its maroon instead of percular?