Here is a helpful hint. Only add ph 5.8 water during the week between reservoir changes.
So, add fresh nutrients, and when you need to top off; Use ph 5.8 water. Leave it for 20 minutes and ph should become stable. Check it and adjust ph if ogg more than .2 i.e. OK if between 5.6-6.0 Never add more plain water than reservoir holds. IN a 5 gallon bucket, you use approx. 4 g. So you can never add more than 4 gallons of plain water before refreshing with a fresh batch of 3 part.
If you run into a situation where the plant is drinking more water; It is time to apply fresh nutrients.
Helpful hint #2 If your mix is too high ppm; your plant will drink more water and ppm will rise. If solution is too weak; The plant will uptake more minerals resulting in a lower ppm. Just adding this to the lesson.