I have travelled in the Amazon and measured the pH and conductivity and a few other things in Tefe, Manaus ( the Amazon 'proper'), on the Rio Negro and Xingu near Belem
The HIGHEST conductivity reading I got anywhere was 14 uS. The lowest pH was on the Rio Negro, 2.8!!!!! the highest in Tefe, 6.5.
These perams are impossible to recreate in an aquarium, but basicly it tells you that there is hardly any mineral content at all, ie, NO salts.
If you use a remedy when it is not needed then it will be less or ineffective if/when you do need it.
Rays handle salt fairly well up to 2 tablespoons ( or more sometimes) per 5 gallons with no problem, for limited periods of time.
All the salts that your ray needs ( electrolites) are already in your tap water