Ok, first we need to get your water healthy before you can start treating. So, we need to know your numbers asap. Take out a sample of tank water now & set it aside to be tested. Then, I would do 50% water change right now with tempmatched, properly conditioned water. Then, change another 50% in a couple of hours/this evening. Remove another sample of water tomorrow morning & take both samples to be tested. Do another 50% water change tomorrow morning, too. Let us know what both samples read (exact numbers, not 'ok' or 'safe' or whatever).
Today/tomorrow, pick up either a large box of aq salt or kosher/canning salt (from grocery store) and you can will be able to start treating once we are sure your tank has healthy numbers. We can through how to do salt treatment once we have your numbers. Keep us posted!