"The fish were boiled in the lake due to the release of hot water from the Mid-Urals Power Plant," Vyacheslav Karimov said.
The farmer said the fish deaths have cost his business 72 million rubles ($3 million) and that it will take five or six years to restore the fish population.
However, Sverdlovsk Region's environmental regulators said the plant had not breached any laws.
"Iset Lake is used as cooling water for the plant," a spokesperson said. "It seems that businessman have chosen the wrong place for fish farming."