[Peter Finke]

Jacob, since this is a Parosphromenus-site and not a Sphaerichthys-site, I cannot exclude special problems with the latter. On the other hand, this is unlikely. But Sphaerichthys are known to be extremely sensitive to skin parasites, and your descriptions, if correct (some good photos would be helpful), seem to indicate a series of subsequent events: first a small lesion, followed by a bacterial infection, and now a funghus. Such a series is highly probable with delicate fish, and the first cause may lie well in the past (before swimming in your tank). This happens rather often by the massive environmental changes (going through several tanks in the import stations and in your tank, at last).
But it is difficult to tell now the best remedy. If you treat the funghus, you may succeed with that but may not act against its causes. If you take measures against those (probably ich or Oodinium) you may not heal its cause and surely don’t treat its consequence, the funghus. If you use several medicines at the same time, you may stress the fish too much and risk antagonizing effects. If you do nothing you may risk secondary infections with the other fish of the group. So what would I do?
I would separate the fish by not catching it with a net but with a glass catching item in order not to touch the fish’s skin. Use strictly the same water by transferring it from the old tank into the new. Then, if it proves to be funghus, I should treat that with a funghicide sold in your aquarium store. If the symptoms disappear, wait and observe. The fish may recover. If it doesn’t, you must treat against the basic infections, but first determine what it is.
If it proves to be Ich or Oodinium, try to get Tetra Hexa-Ex or JBL Spirohexol. If you don’t get it, try a veterinary or a pharmacy, and order “2-amino-5-nitrothiazole 97%”. This is the active substance contained in the two named before. A producer is called Sigma-Aldrich Logistik GmbH, Kappelweg 1, D-91625 Schnelldorf (Germany). The product referece is 133507-25 G. I am pretty sure that there are other producers, too, and, of course, in the U.S, too.