It's not nudibranchs, you would see bite marks and small patches where they were eating the coral and you would find some eggs underneath the coral or next to it. You would also see them at some point during the day or night. My guess is nopox caused a severe drop in PO4, but also check magnesium, super low mag will cause monti some issues. If you're going to carbon dose, then check out the Tropic Marin products, they are very good at what they do and they're not too aggressive either. Their whole product line is geared around your PO4. Also, I'll give you some cap when you're ready.