Hi Denniszz, sorry for ur loss. Do you acclimate your fish and monitor them before releasing it to swim around? I recommend an isolation box for you to monitor the fish's response to feeding before releasing them. A sudden introduction to a tank environment is actually stressful for a fish so do consider introducing them slowly. A fish dying overnight after seeing white patches may not be due to ich though, and suspect it is more from either suffocation (lack of oxygen either in the water or their gills being choked with slime) or stress.
As for food, I find Dr G's anti-parasitic caviar the best at treating fishes so far. It can be found at Ah Beng or Iwarna at Pasir Ris.
Sometimes fishes die also because of the transporting which took a toll on them. It is not recommended to buy a fish which just landed at the LFS as the handling of the fish is rather rough.
As Yono pointed out, do check the parameters of your water too. Ammonia poisoning is lethal and injures the gills of the fish.
Sofishticated Friend