Is your tank 4ft or 1ft? or you have 2 tank? So which one is the problem???
IMO, Dosing sugar may not neccessary bring down NO3....if it really does, is just a short period of time as sugar increase Bacteria metalbolism but after that without sugar, your NO3 will be back haunting you. I tried, but guess not so good....
If the problem is on your 1ft, changed the entire water....to prevent any more losses.
If it is on your 4ft, still suggest changed water...by the way what salt you using? Checked the salt contents.