i have a 4ft tank with 2 8 inch oscars in i done a water change can i add some api aquarium salt and will it hurt my oscars i got told it helps the electrolytes that fish need. filtration in my tank is a fluval 405 external and a fluval 4 internal my tank readings are ph 6.0 ammonia 0ppm nitrite 0ppm and my nitrate is at 20ppm at present time but can go up to 80ppm to 100ppm in one day any advice welcome