yes ur fish are still infected, but are immune or strong enough to fight it off and prevent the whitespot from getting to catastrophic level. the white spots are still attacking ur fish, just very minorly only. if u add in new fish, confirm it will get white spots and u will have to slowly get it immune to it. no point treating the new fish coz its not the new fish, its ur tank and ur other fish. the only way to solve this problem once and for all is to catch every fish out, cure them, and let the tank run fishless for 6 weeks. but its too troublesome, so just make all ur fish immune. best way. but harder and slower. new fish will get the white spot very fast/ but after 3-4 weeks, they will auto immune to it already. seen this happen to all my fish.