hi everyone,
I've been experiencing the same problem and lost some frags over last several months. First, polyp stop expanding, then tissue loss. Algae would grow over the skeleton and then the whole frag is gone. Mine did not RTN but more like s STN (slow) It is quite frustrating
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John from J&L suggested that it may be due to temperature flutation. I kinda doubt it. Mine reading is between 79-82f. Another cause could be low pH at night: my pH gets as low as 7.6 at night. For the last two weeks, I've been dripping Kalk at night to keep pH above 8.0.
Also, I've begin to test Mg level in the tank. I found out that my Mg was only 900 (using Salifert). Natural seawater Mg level is 1200-1400ppm. I tested Instant Ocean makeup water and found only 900ppm Mg. I've been adding Kent Marine Tech M to raise it up to 1100ppm. I have some "micro" frags that I am trying to save, and they are showing new growth after I adjusted my Mg level. I think Mg was most likely the case. Test your Mg level and see if this helps.