If you drain, clean and remix the nutrients every 7 to 10 days, it’s okay to top off with fresh water daily. As plants consume nutrients and water, the nutrient strength in the hydroponic reservoir will change. GENERALLY, nutrient strength should run between 800 to 1500 parts per million (ppm). Your exact ppm requirements will depend on your circumstances and style of growing. We strongly suggest that you purchase a nutrient testing device and start monitoring your nutrient solution’s ppm. We use meters with a “sodium chloride” scale. Before using your meter, calibrate it with General Hydroponics Standard Reference Solution. Then, measure the ppm in the solution. If you find that it is greater than what you want, add water to bring the indicated ppm down to a safe level. If you find the ppm is too low, add nutrient solution to bring the indicated ppm up to its desired level. Your water quality will affect your final readings. When in doubt, remember that it is always better to apply too little nutrient than too much. Unless you know the specific ppm tolerance level for the plant you are growing, it is best to keep the nutrient solution between 800 and 1500 ppm.