When an outbreak occurs, no matter if the treatment is natural or meds, expect to sustain some losses. Thank goodness apparently only one tetra is sick. If i' m not mistaken the Pimafix is also used for fungus. As for your main tank since the other fish appear normal, I would do the noted WC
and just dose once, not all week. I'm not an advocate of Aquarium meds in general but understand on occasion certain meds might be needed. regarding aquarium diseases, the best offense is actually defense. Clean pure water, well vac' d
Gravel, and a constant appropriate temp. Usually are enough to ensure the rarity of diseases.