Some Canadian guy flooded his basement and, after a couple of deaths, successfully maintained a group of monstrous fish, including a few
Arapaima. They can reach lengths of 15' if I remember correctly.
However,
Pangasias gigas is the heaviest freshwater fish. I can't remeber the max weight recorded, but they reach lengths of almost 10'.
Neither of these even approach the lenght of the largest brackish-water fish, which is the Caspian Sturgeon, a non verified account states that a 29' female was caught, and she weighed (if I remember correctly) almost 2000lbs. She was soon gutted for her eggs.
Sturgeon are reputed to live for centuries. They are actually quite common where I live, and a lake that my family owns property at is supposed to have a monstrous Sturgeon, centuries old, living in it. However, the lake is also supposed to be bottomless.
A quick search of FishBase.org reveals the true size of B.C. Sturgeon.
Acipenser transmontanus, 1700lbs+; 20'+.
-Lynden