Interesting. These seem longer and less dense than what I’ve encountered before and thought were hydroids. Could be a more zoomed in image than I’m used to. Again, not saying you’re wrong. Not sure.
I think you’re over thinking g it a bit. You don’t need them to have no phosphate left. Just get most of it and you’ll be fine. Tap water can work too since you have very good tap water.
You do want to cycle them/get them full of good bacteria from your existing tank before using them though
I’ve seen blue color in deep sand before and assumed that I was seeing annerobic bacteria. Maybe there’s multiple types. I’ve seen some black before too, but not usually in active systems
I would dilute 1:5 and add that same 4 ml but do 1 mil every hr. Same total amount but slower and steady. Little bits at a time can fully react and be skimmed out better than what you are proposing
I saw increased polyp extension on acros when I was Dosing vinegar for nitrate reduction. My mind story was the corals were eating the increased amount of bacteria I was feeding with the carbon
Not sure how or why lanthanum chloride “didn’t work”
In using it on multiple systems with what I would call great results. Dosing pump adding tiny bits of diluted solution directly into skimmer body