man Im sorry you are having that continual problem, even after all your remediation attempts.
This happened to me once like it did you, and all I did was drop in a submersible UV
sterilizer and it never came back.
didn't kill algae already on the rocks, but it kept it from ever coming back because we also changed the bad water parameters that let it get hold in the first place. I know there are ideal biological controls to prevent reinfestation...but after those didn't work, a good ole fashioned UV
cheat made it like it never happened...just another option for you, 99 gallons is too expensive to have to keep tearing down, try the cheat!