Immortality Fish

This flat, bottom-feeding fish vaguely resembles a finless sole, but with a key difference--while the ordinary sole produces a chemical that irritates gills (and repels predators), this creature's fin secretions produce a chemical that prevents and even reverses aging. 
They are rare to the point of being legendary even among Illuminated personnel--it's said that they've only been caught a few times, and that siphoning the chemical from the fish also kills it.
== Theories ==
* Magical Modification -- Perhaps the Immortality Fish was imbued to produce the immortality venom, like a magical equivalent to making goats produce silk threads in their milk.This was big news in the realm of genetic engineering, back in 2010! https://phys.org/news/2010-05-scientists-goats-spider-silk.html
** Perhaps some Thaumatological rules allowed one to mix the "unaging powers" of lobsters or certain jellyfish
** Maybe this concoction does make you more long-lived, but makes you a bit more fishy over time, resulting in some New England writers from making some assumptions...See also, Deep Ones
== References ==
* Warehouse 23, p.97