If you're immune suppressed, exhausted or unlucky, it is possible for you to catch Mycobacter marinum, fish tb, from a fishtank. All you need is a severely infected fish that has just died (the bacteria bail and look for a new home) and an open cut.
It shows as it does in fish, with rectangular, hard plaques. On us, they follow the surface areas of the lymphatic system. Untreated, they can burst, in a form of noxious Betta acne.
The bacterium can survive bleach, so in a fishtank, it is hard to get rid of. We're too warm for it, so it barely hangs in and can be killed off with a 4-6 month treatment with a cocktail of antibiotics. It's rare for a person to get it, but trust me (I'm special), I did.