Sounds more like they wanted to sell you stuff than help the fish.
Any chemicals that get added to the water can harm the fish. Adding a combination of chemicals will do harm, and adding salt on top of that, can do serious long term damage to fish.
Maracyn is an anti-biotic and improper use and mis-use of anti-biotics has lead to drug resistant bacteria that kill birds, fish, people, animals and reptiles.
Anti-biotics should only be used as a last resort to treat known bacterial infections that have not responded to normal treatments.
If a shop is telling you to use anti-biotics in a quarantine tank with healthy fish, they are giving you bad information.