I've been keeping Northern DBT in fresh water for years now without any problems. I do add some salt to the water, but that is more as a preventative against infection.
I add salt to all my turtle's water.
I must add to this, all my DBT's are captive bred and hatched quite a few generations removed from the wild. My originals were bought in Hawaii.
I keep my water impeccably clean. Do water changes about every 8-6 weeks. Massive filters.
They also eat like pigs. If I fed them 10 times a day, they want to eat 20 times. All they do is beg for food.
I've yet to have any kind of problem with my DBTs.
Just make sure you buy fresh water raised specimens.

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