Hmm, looks like more than 1600 gph to me. It does look like it is doing quite a good job of creating flow throughout the tank since you have your rocks designed to work with the single powerhead well. Good planning.

My "official" guess then is Xenia Syndrome (my own term lol). Sometimes Xenia melts for no apparent reason, other than the tank has aged. It seems like Xenia prefers new tanks, although this is not always true. The rest of your corals should be ok with a couple waterchanges and some fresh carbon.

Thanks :D I planned to be less packed with LR and more open space. More fishes :D :D

Yah I guess it was just a course of nature where cure has to be deployed instead of pondering over what caused it. I did a 15 gallon wc and another 10 gallon today and have carbon in there. Things looking much better today with SPS having polyps out.