Abstract: A batch process for biologically cleansing water, particularly swimming pool water, by promoting growth of biomass therein, which causes a depletion of the nutrients necessary for future biomass growth. Nitrates are one of the key nutrients necessary for the perpetuation and growth of biomass, or algae, and the object of the process is to promote growth of the biomass until the nitrates have been depleted to an acceptable level. When the nitrate levels reach an optimal level, the biomass is removed from the water, without oxidizing or destroying the biomass, leaving clean water with low nitrate and nutrient levels. This process allows water to be cleansed of the very nutrients that are necessary for algae growth, making the water an inhospitable environment for algae and keeping the water clean and clear. Chemical treatments will require significantly lesser amounts of chlorine and other chemicals required to maintain clean, clear water, after the application of the instant process.