the fear is that you'll have an ammonia spike after using the chemical and kill off a couple grand in fish and coral.
you, and the OP, have nutrient issues. that LFS
gave you bad information. they should have told you to remove the excess nutrients instead of just removing a symptom of the excess nutrients.
the reason i used the chemiclean before was because it was on a public display and they did not want to wait for multiple water changes to take effect. ultimately, it was the water changers that prevented it from coming back, not that i killed it off for good.
i imagine eating yogurt after taking antibiotics doesn't have an immediate effect. I assume any bacteria you pour into a tank will have a dwell time also. if pouring bacteria into a tank was so effective, why do we cycle tanks?