Sorry to hear about your clown and those LPS were looking pretty sickly -- although that favia pic looks good now.
On the GAC, Bag it. Rinse it in RO water or tank water. Drop it in a high flow area like near the return pump or the filter sock if you run them. If you're still concerned about contamination, grab a poly filter and put it in a similar location.
Good advice from
@Diesel. A tasty beverage is sage advice in reef keeping.
A couple other things to think about:
1 You mentioned having moved the corals. If they continue to struggle in a new spot consider putting them back.
2 Those LPS don't need much light; I see a few on the bottom, but don't know your lighting, schedule or intensity. What is your fixture and schedule?
3 There are different schools of thought here, but running alkalinity much over 9 can get a little tricky for a newer system. My experience anyway. Just seems to magnify the impact of other inputs. You could consider letting it drift down a little. Are you dosing two part to get that level or just running an element rich salt? SLOW change if any.
4 Lastly -- and everyone else on this forum has done this at least twice themselves so there is no judgment here -- 3.5 months is a young system for some of those LPS. I am guessing 90% of my fatalities took place in the first 6 months of my last four builds.
Stock the fridge & good luck.
