Safer® Hydro FAQ: How Long Does Nutrient Solution Stay Fresh?
When you mix up a batch of fertilizer and water for your plants, you should test the pH to be sure it's in the healthy range for your system and crops.
Within a few hours of mixing, the solids in the nutrient solution begin to separate from the fluids. Further, as the solution is exposed to air and interacts with it, the pH of the liquid gradually changes. So, for best results, you should mix only the amount you will use in the next day or two.
If you have a reservoir that has only limited exposure to air, you might get away with keeping your nutrient solution for several days, but it's best to stir it up each day to be sure it's thoroughly blended and to double-check the pH at the same time to be sure it hasn't changed too much.
Adding an air pump, such as an aquarium air stone, can be an effective way to limit nutrient separation in a closed-reservoir system.