can become a persistant problem over time. Generally speaking, it occurs from three main areas...and over-stocked tank, and over-fed tank and poor source water. Doing water changes with a well aged SW
mix free of NO3
will help to reduce. Frequency will depend on how high your level is. Reducing feeding to once light feeding every two to three days will also help. Adding more biological filtration such as LR
is one of the best things you can do to keep NO3
Now...we need some info from you. How high is "high"? What size tank? Livestock list? How old is the tank? What and how often do you feed the tank?