Hi hobbyists. My tank is 18mths old. But up to 4yrs old if u count the previous tanks it was upgraded from. Everything has been running well, with good growth and most of the corals colouring up. But this week, I noticed RTN on one of the oldest SPS (acro) colonies, plus another mini colony (acro) elsewhere in the tank. The only variable during this time was the usage of cheap coral chips in my CR, instead of proprietary product. A sifu reefer suggested it might be pH swing. So I went and bought a probe (I've not checked pH all these years). As I suspected, it was not pH, which was within acceptable levels and stable through the day. Madpetz suggested it might be "old tank syndrome". So I cleaned the sump (as much as possible without ripping it apart) and did a major WC. The RTN did not stop. The mini colony is all white and dead now. But I managed to save the bigger older one by fragging. The first sifu then suggested it might be the Fauna Marin salt which I'm using. Apparently a couple of other reefers using this salt have experienced inexplicable RTN/STN too. What do you all think? If salt is the problem, then it's very worrying because this is the most expensive salt on the market and many have used it with good success. I've also read that RTN can be caused by bacteria or virus. And also sometimes colonies die because of competition from other corals in the tank. (I've had a reverse sunset monti die on me after a period of good growth. Then it grew back with a vengeance from a sliver that remained.) According to all literature, RTN/STN are still very much a mystery. Would be good for reefers - esp the old birds - to share their thoughts here.