Hello. I change my hydroponic water once a week. The plant takes different minerals out of the nutrient solution at different stages of veg and flowering. I check the ppm and ph daily, and adjust the ph daily. I like my part per million (ppm) between 1000 and 1400 in veg and 1600 to 2000 in flowering. I keep a mixed 5 gallon bottle of half strength, ph adjusted, solution to top up the reservoir More 2000 ppm and you can get nutrient lockout. Please get a ppm meter and don't just guess. Good Luck. Chuck.