Best thing to do with an impending crash is get two buckets---the living into one, the dying into another in hopes a couple might live in clean water, and the dead into a bin for disposal. RUn your tank skimmer, run carbon, do a 30% water change if you can, and just keep it going. This leaves the tank chemistry /bacteria to recover their balance without a cascade of dying things demising and decaying. Often the tank will pull back from the brink within hours, depending on what caused it [if a pollutant of some sort, a bigger problem] and the living and ok can go back in. Main thing, equivalent to the oxy mask on a plane in trouble---get critters that could die out of the tank and let it recover.
The takeaway from this is: keep a bottle of Prime (so you can 'condition' tapwater for use), a mixing pump of some potency---enough to completely mix new saltwater within 6 hours, which is one very strong pump, a container to mix in, and always, always, always, enough salt to change out 50% of your tank system. THis gives you enough for a water change and a rescue tank.
Running carbon or Polyfilter is a good idea in any tank crisis. So is increasing oxygenation as much as possible.