Ammonia spikes tend to happen shortly after a tank is set up and a fish is introduced.. continue to check it periodically for the next couple of weeks, and when you see an increase in ammonia like that- do a small 10% water change to bring it down.
Also keep in mind a lot of conditioners, such as Prime, will turn ammonia into ammonium, which is safe for the fish but registers as ammonia in tests.
You say turned.. did the one he did previously turn color, or is it new? White stringy doesn't always mean parasite.. keep an eye on him and see if it happens again before you put him through treatment.
And yes, especially at places that buy their fish from mass breeders, you will find fish with all different types of illnesses- including/especially parasites.