Replace if the existing noodles in your aquarium's filter are starting to physically break down; this crumbling or disintergration of the noodles will greatly reduce the biological filtration of your filter.
Be sure not to replace all the noodles at one time - replace 1/3 at a time and leave a week (7 days) between replacement portions.
One 500g box would provide approx. 1 complete noodle change.
