It doesn't matter who or where you get fish from, they should always be quarantined, and not only because of where they came from.
Over time your current fish may have developed a semi-immunity to something normally in your tank, something that the new fish aren't at all immune to, and if the new fish get it, it could overwhelm the tank into an all epidemic.
This is why whenever I get new fish, I add some water to the quarantine tank from the tank they will be going into, every day with water changes. In this way, if there is an infectious agent in the tank, it gives the new fish time, to build an immunity, but not be overwhelmed by it.
Over time your current fish may have developed a semi-immunity to something normally in your tank, something that the new fish aren't at all immune to, and if the new fish get it, it could overwhelm the tank into an all epidemic.
This is why whenever I get new fish, I add some water to the quarantine tank from the tank they will be going into, every day with water changes. In this way, if there is an infectious agent in the tank, it gives the new fish time, to build an immunity, but not be overwhelmed by it.