i experienced alot of problems using NSW as well, in the long run i swtiched to ASW because the amount of manipulation i had to do on the NSW was almost as expensive as ASW use. I do allow my topoff r/o to remineralise before it is added to the aquarium by running it through aragonite otherwise i find the r/o eats up your kh faster. IMO if you not using the same tropical type seawater and you are using temperate Seawater the nutrients in both are quite different
My r/o evaporation topoff goes through an old kalk stirrer filled with aragonite before it enters the water. R/o water is fairly "aggressive" as it has quite abit of its mineral contents removed through the r/o process and it readily wants to react with minerals again so its going to use up carbonates in the process when it comes into contact with them, now we normally want our corals and coralline to use the carbonates more than we want our top off water too.
i dont know if that awkward explanation helps
Have you completely stopped using NSW? I remember that you were a big proponent of it. So now it is all ASW with RO/DI to top off. Could it have been that you were using tap and NSW. I did that and oh boy did my system go down. Hair algae, cyano blooms and coral recession.