The best thing to do to avoid introducing ANY new problems into a tank is to quarantine any new fish for at least 2 weeks before introducing them to an established population. A quarantine tank can be a rubbermaid tub with a filter hung on the side - it does not have to be fancy. Remember, though, that if the water parameters slip - the tank is not cycled - nasties, such as ich will pop out. It is essential to keep a quarantine tank's water as pristine as possible.
I recommend quarantine for all new fish, in a separate tank or tub, with the water at .3% (point three percent) salt. If anything is seen during that time, it is then treated and quarantine is continued until 2 weeks have gone by AFTER the problem was solved. Sometimes this means a LONG quarantine.... so be prepared.
In an established tank, particularly one with a gravel base, ich orgnaisms can encyst and hide in the gravel. There, they can sit and wait for up to years, waiting for the fish to be stressed in any way, or for a new fish that is stressed to come along, and they attack. It may be that you have ich in your tank. Your present fish are strong and healthy at this time, but if you let the water quality slip, or you add a new fish that is stressed, the ich opportunistically will attack.....
Quarantine will not solve this problem, but it may help the new fish become stronger so it can resist problems when it is added to the main tank....