What is the concentration?
What test are you using?
It is as basic as nutrients in and nutrients out. You have too much nutrients going in and/or not enough going out. Sources of too much going in: using tap water for topping off or making water instead of using RO
, overfeeding, feeding low quality food, feeding too many supplements and other foods (zooplankton, pytoplankton, etc.). Not enough nutrients goin out is what happens without enough water changes. A refugium can also allow for additional output, as will a media reactor with GFO
The best first step is making sure there is nothing major causing the input (like using tap water), while doing water changes much more frequently, but not larger.