Your ph crashed. During the nitrification process, your beneficial bacteria use up the buffers in your water to convert ammonia & nitrite to nitrate. The buffers maintain the stability of your ph & prevent it from falling. At a ph of 6.5, nitrification slows down. At 6.0 ph, nitrification stops completely. You will need to do a big water change (90+%) with temperature-matched, conditioned water to bring your ph back up into a range the nitrification can occur & to re-establish buffers to your water. You should also set some tap water with an airstone or bubbler & leave it for 24hrs to gas off- this will give you your true ph. The salt isnt necessary for a fw
tank & really serves no purpose here. Dont hesitate to ask questions!