I agree, but just because a fish flashes means it has a parasite, unless it's the same fish doing the same spot. I would suggest just using aquarium salt and conditioners, such as Stress Coat. These help the fishes coat. The salt helps kill parasites, becuase they do not have the body function to process the salt. I would not use meds in a tank, unless it's a QT tank and you know you are treating the fish exactly for that problem. Most meds kill bacteria, good or bad. Therefore, you kill your benificial bacteria and you could get into more problems. With no or little bacteria, the ammonia/nitrites will spike, almost similar to a new cycle. Most meds also change the water parameters, which could stress the fish out.
When using aquarium salt, never just pour the salt in the tank. Put some in a cup with water, then pour into the tank. Do a little at a time, until you get the proper dosage for your tank. I use a similar method and even my clown loaches, who are sensitive to salt, do well.
