OK, addressing the decent amount of water and corals using little O2 points:
Using an Acropora respiration rate of 0.2 gO2 m^-2 hr^-1 (derived from A. palmata), and an O2 solubility of 6.6 mg/L at 25C and 35 psu salinity in water, if we place a 5 cm x 5 cm piece of coral in 1 L of oxygen saturated seawater, after only 13 hours, less than 2% of the original mass of O2 in the water will remain! That's 100 millionths of a gram of O2. In an entire liter! Proper gas headspace and sufficient oxygenation of that headspace is a MAJOR factor!
That said, these pictures make me ill. Those corals are incredibly poorly packed and probably looked about the same yesterday.
You chose one of those most o2 demanding corals :lol:
Don't forget, the plastic used is o2 permeable to a degree, no bags we use block o2 transfer entirely.