The largest fish live in the ocean and cold water lakes. For example, the Whale Shark, Basking Shark, Beluga, and Great White. These, I think all range from 3 to 13 tons.
I think it's just that there is no evolutionary advantage to being 3 tons in a large river (e.g., Amazon), or a large lake. You have less oxygen (cold water holds more oxygen) and need more food (with higher temperatures.) In shallow lakes and slow moving shallow rivers, rapidly peaking summer afternoon temperatures drive small fish to the surface. It would be far worse for a 3 ton fish.
In trout, as an example, in one study I saw, their 'optimal' temperature for growth and efficient food usage is 55-65 degrees. At 68 degrees, they hit a limit. At higher temperatures, they don't feed as efficently and they encounter respiratory distress. Since all FW fish are descendants from SW fish, it's plausible that this scenario is the same for all warm water FW fish, although they are adapted to a warmer temp.
In cold water, there is never an issue. If fish get to 3 tons, they are efficiently using food and suffer no distress from lack of oxygen since coldwater environments (oceans and the largest lakes) are extremely stable temperature wise.
Warm water fishes need to be able to adapt to temperatures that might fluctuate from 68 to 90 in their ecosystem, albeit over the course of 24 hours.
