Hope this helps, To do this, empty or drain 50% the total volume and replace it with fresh water (example: if your reservoir holds 100 liters, and over a couple weeks your logs show you’ve added 50 liters by topping up the water, it’s time for you to take out 50 liters and add the same amount clean, pH balanced water). Most hydroponic systems need this larger water change as frequently as every two weeks, although smaller reservoirs may need it as often as every week to ten days. You will also need a daily or every couple of days top up of water before a whole reservoir change. 🌱👍