I am not sure how the salt will effect anything in your growbeds. So think about that before you do anything.
I start treatments with salt at 1 tablespoon per gallon of water and watch for improvement in symptoms/body language/gill labor. Tilapia can tolerate high concentrates. I have read where they are working to grow them in brackish to salt water conditions. My MAX is 3 Tablespoons per gallon.

Dissolve in tank water before adding to main water. Make changes slowly if you decide to go this route.
I keep mine fairly "salty" all winter and completly fresh during summer. Salt does aide them in wintering in my systems.