Quote from "Culture and Propogation of Striped Bass and it's Hybrids" edited by Harrell, Kerby and Minton states the following:
"It is generally accepted that water with a total hardness greater than 150 ppm is very good for phase I culture. Under hatcherty conditions, excellent phase I crops have been reared in fresh water ranging in hardness from 60-600 ppm".
"Stress problems resulting in mortality have also occurred in striped bass and hybrids when handling fish transferred from hard water to very soft water. Hybrids are somewhat more tolerant than striped bass to soft water, but should not be subjected to radical changes in water hardness".
That said, my hardness in all of my ponds ranges from 400-480 ppm so it's not an issue for me. I couldn't find any other references as to minimum hardness needed for HSB survival.
Holding a redear sunfish is like running with scissors.