Help with Torch Coral

Hi,

My torch coral is in trouble. I lost one head completely a few weeks back - just disappeared until only skeleton was left. I cut that head off and now I have one head that is not extending as far as the other two and is much lighter in color?

Tank/Water parameters are :
250 Liter's
pH - 8.1
Temp - 79
Amm - 0
Nitrate - 5
Phosphate - <0.03
Calcium - 375 ppm
Magnesium - 1345
Alk - 9 dKH
Salin - 1.025

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