You really should be changing out the water more often, different stages eat more food and nutrients become unbalanced, u can add nutes in with water, but it's advisable to change the water at least every 2 weeks, if u use less water i would do it once a week. Depending on ur set up u can go very long with no res changes, aeroponics would be the longest, dwc should be the shortest. Keeping this balance can be a huge advantage. Watching your ppms is a big factor, keep an eye on how much water transpires and if the ppms drop, normally adding in the same ppms is a good idea, but at some point your going to hit an unbalance.