Just because a fish goes to freshwater at certain times of their lives doesn't mean it's ok to keep them in freshwater always. A Columbian shark is born and grows up in freshwater and then moves to saltwater when older. To my understanding people keep Columbian sharks in freshwater when adult, but they get sickly and usually die. Striped bass go into freshwater at certain points in their life. From what I've read the freshwater lakes they're stocked in have a noticeable salinity level from minerals in the rock where the lake is. The lake 1 mile from me is 12000 acres but doesn't have stripers, because it's completely fresh. A lake 50 miles south of me has stripers, because according to their information the salinity level allows them to flourish. There's no way to know for sure if keeping a bumblebee grouper in freshwater long term hurts them in any way. If you're willing to take the chance anyways I don't see how you can care about the well being of the fish.