Reservoirs should be emptied and the nutrient solution replaced at least every 5-7 days. People often use their EC/TDS meters to measure nutrient levels and top up their nutrient solution when levels get low. EC/TDS meters only measure overall salt levels, not the levels of specific nutrients. This means nutrients not completely used by your plants will begin to create bacteria, which can cause algae. The solution to this is to turn off the pump, empty the reservoir, clean it and fill with a solution that has been left to reach room temperature.JUMP TO HYDROPONICS