Once took a semester long course on ichthyology, a long long time ago in a galaxy far far away.
Fish living in freshwater have a challenge where their blood have a higher concentration of solutes than water in its external environment. Bodily processes of freshwater fish are thus always countering osmosis since water has a natural osmotic tendency to enter their bodies while salts have a natural tendency to exit their bodies. In general, freshwater fish physiologically osmoregulate by retaining as much salts as possible at exchange sites (gills, gastrointestinal tract, urine, skin) and expelling water. Freshwater fish are basically always peeing water to maintain homeostasis. This is why adding a bit of salt for FRESHWATER fish eases their energy expenditure and helps when they are under stress.
In contrast, saltwater fish have a lower concentration of solutes in their blood than natural seawater. Water thus has a natural osmotic tendency to leave the fishes' bodies while salts have a natural osmotic tendency to enter their bodies. Saltwater fish are basically always drinking water and peeing fluids with high solutes concentrations, and expelling salts through their gills. This is why lowering salinity for SALTWATER fish helps when they are stressed, as it eases their energy expenditure in maintaining homeostasis. This does not apply to Elasmobranchs (sharks, rays, ratfishes)... their blood have higher solutes concentrations than seawater.
So if you go HYPERsalinity, it's going to energetically tax the fish more so than their natural environment.
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