I hope that I am really wrong with this guess.

I might also add, if you have a cut, let it heal before you put your hands in that tank. If one cuts their hand while working in or around a tank, let the cut bleed clean over a sink and squirt a little hydrogen peroxide over it.

While the Mycobacterium species which cause fish TB are not the species which cause human TB, you finger or hand can be infected with these species which are most comfortable in the 70s or 80s F.

If you do a search for "Fishkeeper's Finger" you should find several hits. Even the AMA Journal and Good House Keeping have had articles on it as early as the '70s.

Infections can be cured with anti-biotics, surgery for those who ignore it while it spreads on one's skin. Elderly people may find the path to a cure will involve even months of treatment because their immune system doesn't combat it as effectively as with younger people.

Several years ago, I developed a crusty skin patch on my hand. One big honking dose of a new (to me) anti-biotic for a sinus infection also cleaned up my hand! Guess i was lucky.

Mycobacterium are common in tanks. It will exist benignly in the mulm at the bottom of a tank for years. When water conditions slip or an older, less healthy fish contacts it, it can enter it's more virulent and even more contagious phase.

Fish TB is virtually impossible to cure with the fishes. You did the wise thing with your late fish.

At least one party has suggested that boiling gravel and bleaching the tank is a start. He then felt that swabbing down the tank sides with rubbing alcohol (which would rupture Mycobacteria cell membranes) was necessary.

The stuff seems to have entered my tanks when I purchased several rainbowfishes from different sources. An infected tank was quarantined and attended to with different equipment. I haven't seen evidence of it for years, but who knows?

See:
http://members.optushome.com.au/chelmon/Myco.htm