Erm I will suggest you buy a Salifert NO3 test kit to test and see if the result is the same or not. If yes, try to do 30% water change every 2 days for 3 times. At the end, if your NO3 still raise up very fast then u need to consider reduce your livestocks or do carbon dosing. Btw do you have alot of bio media? If you don't have enough, your anaerobic bacteria also no place to grow.