Quarantining for two weeks may not be enough. I would say that you need at least a month of quarantine to be safe. Ick can be present in the fish gills or inside the mouth that you can't see. I have the bad luck of an ick out break recently by introducing new fish after two weeks quarantine.I think it was some Heros species I introduced.I always quarantine my new fish for two weeks so I don't really understand where it came from.
I haven't had an outbreak of ich for years.
https://www.monsterfishkeepers.com/forums/threads/chemical-resistant-ick-or-vulnerable-fish.686243/
I have encountered some resistant ick strain out there that is hard to eliminate. I have been treating for a month with MG/formalin remedy to finally believe it is cured. Sting Ray are known to be sensitive to MG, so your choice of treatment is limited salt, copper or transfer method. I am not sure of the safety of copper treatment to Ray, but the transfer method is safe proof but involve a lot of work. Essentially, you move the fish from tank to tank, or do massive WC daily, to physically remove the ick swammers from re infesting the fish.