Sounds like a dormant ich parasite that struck quick and hard when the fish were left vulnerable with high salinity and high nitrates. Either that or kidney failure from the high salinity. How high did it get? The fish could have all had massive kidney failure if the salinity got too high.
. I need to buy some Bio-Spira tommorow. So incase that happens and all the bacteria dies away then I will have some extra in my fridge.
Methinks thats kidney failure range... 
