Any treatment should be done in a QT. Do you have one?
That product will most likely NOT work and will just cause you to delay before starting an actual treatment. I would like to know how and why it would be effective in the first few hours but not after? That just does not make any sense.
There are, to my knowledge, only two known cures for Ich. One is a copper base treatment such as Cupramine which will kill all inverts in your main tank, and the other is Hyposalinity (which will kill all inverts in your main tank). That's why you need a QT.