I should have said, "reported as" biggest freshwater fish ever caught using a rod.
I found this listing of Cryptids online:
Ray: Giant freshwater ray, unknown species
Reported from Mekong River, Thailand and Laos
This bears further examination. I do not know if the above article/photo corresponds with "Sehm". Whoever "Sehm" is, I want to find out.
I'm getting closer...
Sehm, G.G. 1993-1996. On a possible unknown species of giant devil ray, Manta sp. Cryptozoology 12: 19-29.
The Waitoreki of New Zealand - Marsupial or Monotreme? in Tuatara: Volume 30, Issue 1, December 1988
Krumbiegel, I., and G. G. Sehm. 1989. The geographic variability of the plains bison. A reconstruction using the earliest European illustrations of both subspecies. Archives of Natural History 16(2): 169-190.
5 hours ago