If you used Metronidazole and salt, it's unlikely to be an internal protozoan infection. I would try deworming the and maybe increase the salt levels.
How much salt did you use originally and how long did you have salt in the tank with them?
Rosy barbs and swordtails can have 4 heaped tablespoons of salt per 20 litres of water for 2-4 weeks.
And you can use salt with dewormers like Praziquantel, Levamisole and Flubendazole.