I spoke with our consultant about this before, and he mentioned that dietary fat actually helps remove fat from the body, particularly body fat from carbohydrates. He couldn't remember exactly why though.
There is an increasing amount of research being done on exercise in aquaculture. I'm actually about to give a presentation on it. Since lipids are the primary fuel for sustained swimming, I would be surprised if it couldn't help. It improves lipid mobilization in the tissue, so it may work for getting the fat out. Since we are a RAS farm, our largest, oldest fish usually get the slowest moving water since they eat less and need less oxygen. For that reason they tend to get fat. These are fish that we still have because they never produced eggs, but maybe increased fitness could help some of them. When the owner of a Chinese sturgeon farm came to visit he said "those fish look fat, maybe you can make them swim more". There was some interest in testing this but the farm was sold and the interest went away. I would still like to see if it works though!