SO, I did read the whole thread, OMG! lol
So even though I think you can improve light, flow, etc, I don't think that's likely to be the cause of sps death. Contamination that would be another possibility isn't bc you did the ICP test and everything came ok. So, we could go on and on about possibilities. BUT!!! There's something we can see there and we know it kills acros!
IMO it's what I saw in the first video from the first post. That brown slime that covers the rock and later on we see covering you frags, that can be deadly. Looks like some kind of dino, but only a microscope will tell. But doesn't matter if it's dinos or cyanobacteria, that kills sps and we know that. So, first thing to do is to solve that. Not easy I know! I had dinos here and almost lost all my acros (little frags haha). I know of very experienced aquarists that lost many large colonies to dinos. Yeah, it can be a pain in the butt. Sorry, it's not something easy.
Now, I think it's clear the brown slime is the issue here. What to do? There are many strategies to solve this issue. First, solve the nutrient unbalance by raising PO4.
This unbalance makes it harder for beneficial organisms to come through ecological succession. And you end up with an outbreak of undesirable things like dinos and cyanobacteria. They end up with little to no competition and predators in this rush environment.
What I did here to solve dinos may or may not work for you bc there are many different species of dinos (if that's dinos). But I did a 4 days blackout, dosed vibrant (yeaahh, maybe that is rush on acros), dr tim's waste away, niteout, stirred the substrate, added a UV rated for 4 times the size of my tank and went with low flow. I also did microbubble and cleaned the skimmer and socks twice a day. This may not be the ideal treatment, it's just what I heard online and did as I was kind of desperate to get rid of the dinos. So far, I only see brown algae here, no confirmation of dinos. But I don't know yet, they may come back.
And this treatment was VERY VERY rush on my acros. In your case, I would take those frags out. And I did lose 5 anthias purple quem, but that's another story... Acros seems to be recovering here. I started dosing acropower too.
EDIT: I'm far away from being an experienced aquarist, ok?! haha only 4 months since I added fish to my tank and it's my first time doing saltwater. So, I don't have half of your experience. So I may well be wrong.
