Having similiar issues with RTN effecting few corals only - My guess chemical... If I did 3 large water changes my alk would go up too much from 8.2 since IO mixes 10+DKH. Did you use a different salt mix for these change and how do you handle the swings?Unfortunately I had some RTN going on in my tank. 3 different colonies were affected. I sent some water in to Triton, and everything looks pretty normal. I did 3-40g water changes, and the RTN stopped. I wish I knew exactly what caused it, but in this hobby that seems impossible. This was the first time since I started this tank 4 years that I’ve had this happen.
Here’s my triton results.