salt water/shell rot

Hi! Haven’t posted here in quite awhile. Been getting things straightened out with school. Tex still has his shell rot and I’ve done everything I know I can do. I raised his water temp, cleaned the water every day, fed him healthy foods and he has a UV-UVB light. I’m down to my last idea. DBTs get shell rot from not being in brackish water (or atleast are more suseptable) right? He’s a little over a year now so how about I put him in an aquarium with a little salt? I bought that really popular instant ocean stuff. I know the DBTs in the wild go from slightly salty water to a higher salinity as they mature so the question is how much do I put in? The box said 1/2 cup for every gallon. ~Candace