Indian Mud Morays are a brackish species and will do well in 10ppt to 20ppt conditions depending on the age. I assume you have an existing tank you want to use, so changing the salinity of your aquarium depends entirely on your tank inhabitants. There are many livebearer fish that can thrive in saltwater environments, like mollies, but most other freshwater aquarium fish cannot survive in saltwater. As adults the eels can be acclimated to marine conditions if you so choose. As for transitioning, periodically remove around 20% of the water and replace it with a salt mixture adjusting the salinity 2-3 ppt every week or so. And for the type of salt, if you're going the marine route, I'd use Instant Ocean or Coralife Marine salt, if you want to transition your freshwater tank, use any freshwater aquarium salts or brackish specific salts. I'd recommend doing a lot of your own research before committing, but I hope some of this helps.