always getting sick

[hound96]

i have no clue what to do i have tried everything and i am almost ready to sell my tank , i have tried all the methods of introducing fish and still they get ich even when i know they srent sick when they go in my new one was watched for 2 weeks and even when i did put it in the tank it had almost no stress sighns and what it did have was minimal and gone next day, also researching the fish they are extremely hardy and even considered beginner fish but yet here i am tonight looking at white spots on my fish what the F