I’m not sure it’s the ‘high’ po4.
I think it’s more probably all the transitioning your tank is doing.
First very low nutrients with vinegar, then stopping that..the use of gfo and then the rise in nutrients.
I think maybe just lack of stability.
Heavy gfo use in a reactor may cause po4 levels to swing up and down as the gfo is replaced and then gets exhausted..
Can’t rule out metals if you found something rusting, however.
Do you use a fuge? They can be pretty good at controlling nutrients.. and at some metal export..
Or even the vinegar isn’t so bad is used in moderation. Instead of using it to drop n to below, what about using enough to get n to around 5ppm. Po4 is more tricky. I’ve been using gfo in a more passive way lately. I let it sit in my sump that contains my skimmer. There are actually 3 bags of gfo in rotation. The oldest bag gets to sit next to the highest flow from my skimmer. The second oldest bag is in lower flow and the newest bag is in the lowest flow. Each week I rotate the bags - toss the oldest gfo and replace with new gfo..
I know rather labor intensive but quite stable with decent control,
The use itself of gfo shouldn’t stress the corals , it’s how aggressively it’s used.
currently, my n is low at around 1 ppm and my p is finally steady at around .06. I’d like to bring my n up a bit.. I’ll try to do this by reducing the photoperiod of my fuge by a couple hours.
The problem with shocked or stressed acros is that they can take weeks or months to finally die from stress. So when an acro dies, it may be due to something that happened so long ago that it’s difficu to connect the cause and effect.