I personally do not have experience with ich at all, so I may not be able to offer you sound advices.
However, medications would normally come with instructions. Doesn't yours have them? I understand that you would need to follow the full course of medication to totally eradicate them.
The sticky Freshwater/Tropical Fish diseases has some information too. You can also do a search using the keywords "white spot". There are some previous discussions.
Good luck with your fishes.
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