Dead Tetras

[Fremlin]

I recently purchased 6 Emperor teras for my tank and on the next morning I noticed that 2 of them had a slight discolour, one had it just below the dorsal fin and the other at the tail. The next day they were both dead. Anyone any ideas what it is, and how I can prevent any of the others getting it?

 

[WILDER]

Need quite abit of info first.
Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia,nitrite,nitrate, and ph.
What was the ph of the fish store to your tank, and how long did you climatise them for.
The white patches did they look fluffy in appearance.