well, I do not know much about biofilters but At 5 weeks your tank is likely still cycling. This will cause the NO3 to continue to rise until all ammonia and nitrite are gone. Have you tested for those? Results?
What is in the tank? What are you feeding? What kind of water are you topping off with and mixing new water with? Have you tested a new batch?
Sorry for all the questions but NO3 can come from a multitude of sources and depending on the volume of tank and water change you may not even make a dent in it.