To tell you the truth I didn't belive it either until I started searching the web and I found all sorts of infections related to not washing up after putting your hands in the fish tank, in fact there was one case were a person ahd an open cut, nothing terrible but from putting his hands in the tank and not washing up go some kind of infection where this guys finger was like tripple the size,
You are referring to Mycobacterium marinum, which is a known bacteria that inhabits freshwater, brackish, and saltwater habitats...this is indeed a danger while working in fish tanks. As stated several times now though, the dermatophytes that cuase ringworm could not be found in an aquarium, and being a fungus...have nothing to do with the bacteria in a cycling product and would not be there (no available host).
Even though you describe yourself as a clean, lazy hermit....I would still imagine you, in your daily life, have come across something that has been touched by another human being considering you went outside to purchase the product from a public pet store. It was most likely something like this, that you did not pay much attention to, that was the beginning of you getting infected.