If tested nitrate level is 20ppm or less everything is ok. If between 20 and 40 ppm it is getting high but not dangerous. Do 20% water changes daily till it is down to 20ppm. If over 40ppm then do a 50% water change then 20% a day till at a safe level. If high or not coming down test the tap water. Several years ago my City's water had a problem and I was getting 10ppm right from the tap.
good luck