Read up on the life cycle of ich. Read the articles that were given here...
What you describe is the normal life cycle.
1. Little specks show up.
2. Fish may fend it off and heal up the wounds.
3. Now nourished, ich hides away in your tank and reproduces.
4. 7-10 days later, ich comes back in exponentially larger numbers and attacks fish again.
5. Go back to step 1, but with more little specks.
The "meds" you used don't do anything. You could've done nothing, and witnessed the exact same thing happen to your tank.
The ChemiPure is the least of your concerns. You need to get your Tang in a QT, use known effective treatment (hyposalinity or copper, preferably hypo), and let your main go fishless for 6-8 weeks to break the life cycle and starve out all the ich. Save your money on doing the herbal treat again, and just bite the bullet.