A water change is not going to do anything to help with parasites. A UV
is also a very limited item in dealing with parasites. Often being attributed as a cure when in fact it was a part of the larger picture.
A single white spot on a fish does not necessarily mean Cryptocaryon irritans
nor does a black spot indicate tubellarians
. If they are spreading or you notice movement in the "spots" then it would be reason for concern. Tubellarians are treatable as is C. Irritans
but not without a proper QT
. If you cannot leave the main tank fallow, the problem will persist as long as there is a fish host to feed it. Until the parasite is without a fish host for 6 weeks, you will continue to have problems until you do QT
If the black spots on your fish are primarily on the face close to the eyes, it could simpley be from lack of proper foods. Tangs and similar species have pits in the general area around the eyes that become more noticable if in poor water conditions or if not being fed properly fortified foods.
Before you begin any treatment be sure of what you are treating. If possible post a pic. Tubellarians
are best treated with formalin dips and C. irritans
is best treated with hyposalinity or Cupramine. All of which require the use of a QT