Agreed with DaveK, IMO, that's poor advice from your LFS, good for sales, bad for nem.
Ask the members here, will save you time and money. I've killed enough stuff in my early days.
I noticed ever since I listened to members of reef sanctuary rather than store, everything is working so well now and I now have that tank I struggled with for years prior.
My advice (which is the same advice I was given) works for fish, inverts and corals. Take the time to become a water expert. Get the test kits, work them weekly, record findings, you will see a pattern.
When tanks are fully cycled, when using RODI, when maintaining all 8 parameters within NSW ranges, consistently for say 6 months minimum, things put in the system do well.
To that just add good lighting which includes right spectrum and good intensity, esoecially for Nems who love their light.