Looking for help

I have had my tank for about ayear everything was going great and all of a sudden all my fish died ( looks like there fins were eaten off) also all corals shriveled up and are not doing well at all. I have placed corals in another tank and no improvement. I did a 50% water change and then a 35% water chage then put 2 damsels in the tank they also died after 3 days ( looks like fins were eaten off) the only thing that was not effected as 2 shrimp and a crab??? Does anyone have any idea???

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