I believe we can agree that OP issue is not virus.Yes, I know.
This happens regularly, everyday at low tide. But, if it doesn't happen everyday for months on end and then one day it does for 2 hours do you think it will respond the same way?
Looking at the original pictures and reading through the information that OP provided and from my experience my comments are:
1) Zero PO4 /P is bad for new tank,
acros need food and I seen same issue in my tank under the same conditions. STN sets in and the acro deteriorates from bottom up. If you snip the good part it might survive assuming the condition is corrected, but based on the pictures these look like sticks that never took off in the first place.
Hence going forward OP should consider feeding the corals.
2) Keeping struggling /stressed SPS out of water is probably not a good idea. But bviously from the pictures they are hanging on… Also I am going by my experience, I did something similar and things survived. Should you keep sensitive SPS out of water for 2 hours, I am not suggesting that is best plan. But in this case I do not believe that is the main cause, based on my experience and information provided.
On the flip side OP was going to use antibiotics, I think that is not the best idea. Dosing antibiotics into a system that depends on bacteria is not what I consider the best plan, but people do it anyway.

