Well in order to get to the problem and help you find a resolution, we will need some information from you.
What're the parameters and has anything recently changed whether it be minimal or drastic?
Have you inspected the frag for any pests?
Seeing it's been doing fine for a month, I'd assume it's a recent frag purchase, have you dipped it prior to putting it in the tank?
Have you changed anything from flow to lighting?
Do you think it's possible a contaminate of some sort has entered the tank?
Along with the above question, have you put your hand in the tank without washing it thuroughly with warm water? ie: cleaning dishes, or doing something else that may have left harmful chemicals on your skin
Tons of questions will come with such a broad question
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Help us help you.