In the wild TSNs pray on cichlids and other tropical fish and also crustaceans. 99% of them are very lean - they don't need fat neither for energy nor for buoyancy. (Some, rare fish like pacu can have significant fat deposits to survive the dry season in tributaries and seasonal ponds, not so in the main river channels.) Cold-water fish all need fat but to various extents.
Been proven beyond any doubt at Public Aquaria that feeding even such cold-water-but-highly-adaptable-to-warm-water fish as goldfish to tropical predators, f/w or salt, results in fat deposits around their internal organs as their body is not built to process such amounts of fat. The usual diagnoses are "death from a fatty liver disease".