From 'Dr. Aqua':
The disease is caused by a small worm known as a tubellarian. After parasitizing a fish, the worms develop on the fish's skin and gills and acquire dark pigmentation. They are freely mobile and will tend to move over the surface of the fish. After five or six days depending on the environmental conditions, they drop to the bottom of the aquarium. There they mature, with the development of the young worms within their body. Once the development of the young is complete, the adult worms burst, releasing the free swimming young that infest new host fish. The worms can be controlled with various commercially available medications. Formalin based products or those containing organophosphate compounds such as trichlorfon appear to be the best medications. In addition to the use of medications, any excessive buildup or organic material and debris should be siphoned from the aquarium several times during treatment. Since the young worms develop on the aquarium bottom, the removal of debris will aid in controlling the disease by reducing their numbers."
This would argue for a bare qt for both accuracy of dosage and also for allowing a fallow period in the main tank. How long would be sufficient is not certain. Watch for evaporation changing the ratio of water/meds, and change the filter floss obsessively.
Read the how-to sticky at the top. I've made a link to your excellent photos, so perhaps your treatment will help others. Feel free to make notes there.
