FW and SW DO have different nutritional needs
“Marine fish typically require omega-3 fatty acids for optimal growth and health, usually in quantities ranging from 0.5-2.0 percent of the dry diet. The two major essential fatty acids of this group are eicosapentaenoic acid (EPA: 20:5n-3) and docosahexaenoic acid (DHA: 22:6n-3).
Freshwater fish do not require the long-chain highly unsaturated fatty acids but often require an 18-carbon n-3 fatty acid, linolenic acid (18:3-n-3), in quantities ranging from 0.5 to 1.5 percent of dry diet. This fatty acid cannot be produced by freshwater fish and must be supplied in the diet. Many freshwater fish can elongate and desaturate linolenic acid using enzyme systems resulting in longer-chain omega-3 fatty acids EPA
and DHA, which are necessary for other metabolic functions and as cellular membrane components. Marine fish typically do not possess these elongation and desaturation enzyme systems and require long- chain omega-3 fatty acids in their diets.”
However, I don’t know if fresh and saltwater fish flakes are actually different - manufacturers could meet the nutritional needs of both with one product - I checked out nutriton info for Cobalt flakes on their website since they seem to have a formulation for everything. When I compared Cichlid vs marine Omni formula I was surprised the ingredients are identical (they list them in different order though - suss!) but one has a higher guaranteed protein, although that doesn’t necessarily mean there is any difference.
So my advice is - at least check that SW flake brand also makes freshwater flakes.