I was just going through your post and I think I am having a similar thing going in my tank
@Llorgon . For the last 1 year i am battling this.
I am posting a link for the thing I have. And funny enough i was also looking for a microscope just 10 minutes back!
I have put 3 samples in glass . One with flucanazole, Azithromycin and the third being the control.. later in the video i show my tank..and how the corals are affected...
Does that look similar to what you have?
My system is nearly 6 years old and strangely for me, my Nitrates are high around 40ppm and PO4 is 0.04ppm.
I also have a red cyano infestation in some of the walls of the aquarium.