Great pictures, nice to see some corals are doing well!
What the heck could have happened to get those odd burnt tips? I'm throwing out two ideas that popped into my head, but I'm honestly not a fan of either one.
One thing that stands out is using hydrochloric acid (using the more dramatic name here) to lower KH in new water. Since water changes turned things around this doesn't make any sense but I'm wondering if this is a long standing practice or something new with this tank?
My second thought would be the algae reactor. There's a few folks who like to preach about allelopathy associated with algae. Could the ARID reactor be so efficient that the mere quantity of algae in the reactor might be a vector for this to get back into the tank? Of course one has to believe allelopathy is a real issue between algae and corals, which is certainly up for debate.