Why do closed aquarium systems need some water replaced each month?

In closed aquarium systems, water is continuously recirculated rather than flushed, which leads to the accumulation of metabolic wastes that cannot be entirely removed by filtration alone.1

While bacteria in the filter material convert harmful ammonia into nitrite and subsequently into nitrate, plant growth is typically insufficient to consume all the produced nitrate. Consequently, replacing 1 to 10 percent of the water per month is necessary to maintain low nitrate levels and prevent the buildup of these nitrogenous wastes. Additionally, periodic water renewal helps manage the increase in water acidity caused by metabolic processes, ensuring a stable environment for the organisms.1