What your LFS gave you is called erythromycin (or EM). it is a very mild anti-biotic, which allows if to kill the red slime algae (it's actually a bacteria).
You need to follow the instructions exactly or very very bad things can happen. The dosage program we recommended at my LFS was:
1- 1/2 recommended dosage on package
2- 20% water change after 24hrs
3- repeat steps 1 and 2 twice more
4- 30-40% water change.
Depending on the amount and growth rate of the red algae, the number of times the steps 1 and 2 are taken can be more or less, but 3 doses is usually enough to know it out.
The water changes are crucial, as it creates a means for the antibiotic to leave the systems. Otherwise, you add a 1/2 dose, say 10g's. Then another dose, you're up to 20g's, then another, you're up to 30g's. You've essentially tripled the recommended dose without knowing it, which is very bad. The water chagnes also help kill any remaining algae naturally, by decreasing free nutrients in the system.
Also, wait 3-4 weeks before adding any fish (corals are ok). The EM will kill off a small amount of "good" bacteria and you're tank will undergo a small cycle. it's improtant to keep doing good water changes to minimize the cycle, and prevent the red algae from returning. if done correctly, it shouldn't show signs of even thinking of returning for a good 5-6 months.
Other means of killing the algae naturally are increasing flow, large frequent water changes, addition of live rock, and decrease feeding.
Good luck!
