SG st 1.018 is still too high to try to kill the white spots.
You need to lower the SG to 1.009 and keep it there for at least 3 weeks after all the spots are gone from the fish. 6 weeks is much better.
pH crashes easily at those salinity especially when your filtration system is not matured and I guess this is the cause of all your problems with fish dying. Therefore keep the kH at ard 10 will help to stablise the pH.
Sometimes when the fish is too seriously infected by ich, you can try to feed like hell but the ich will take away more than what the fish can eat and end up the fish also up lorry