why can't we culture marine fishes in fresh water reservoirs?

This is because marine fish cannot survive in freshwater.

Osmosis causes the cells in saltwater fish to constantly lose water, which the fish has to replace by drinking salt water, desalinating it with special organs and replenishing the water in its cells

But when put in fresh water, the cells in a saltwater fish would quickly absorb water from the environment, and with no way to eliminate it, the cells would swell, burst and kill the fish.