If it eases your mind at all there is a belief that RTN may have an association with a pathogenic coral bacteria that is opportunistic. When it senses that the coral's immunity is not very strong it is able to take hold and just decimate the coral tissue. However, from my experience in a marine microbiology lab as well as my discussions with people at the Smithsonian Marine Station, the pathogen is not easily isolated. Only around 3% of marine microbes are able to be cultured and identified. On top of that, there are many microbes living on coral tissue that we have no idea what they do. Even if we could manage to isolate the bacteria present when an acropora is RTN'ing we still could not assign blame to a particular bacteria because it is more than possible that it is simply coincidence. The Smithsonian had a problem with hard corals STN'ing and they asked for our help at Harbor Branch Oceanographic Institute and we told them even if we found bacteria we can't guarantee that it was causing the problem. The best bet is reduce as forms of stress as much as possible, with a focus on calcium alkalinity and temperature. Not saying the others are less important but a swing in say a few ppm of nitrate wouldn't be as bad as a swing in 1dkh in my opinion.
