Good advice from ^^^^^ i try and keep mine ranging between 5.5 and 6.5. I have found that plant health is better if i allow ph to range up and down somewhat. Some elements are more available at a higher ph, some are more available at a lower ph.
Z may have covered this in the reference to the Cycle of Life link, i didnt check it out. Also remember that my ph meter might read at 5.5 while yours might read the same nutes at slightly higher or lower levels. Its all about being able to "read" the plants health to find the best ph and ppm. If you run the ph at 5.5 most of the time, and then change it to 6.5, there will be a noticable difference in the general health of the big fan leaves. It might be a slight improvement, ie the fans standing up loud and proud. Or there could be a slight decrease in health, ie the fans drooping and wilting a bit. As the master of the system, you need to be able to determine which changes make a positive effect on the plants, and you also have to be able to determine which changes have a negetive effect on the plants.
Its pretty simple. All you have to do is stand back and "look" at the plants before any change in the system is made. Then about two hours later, go back and "look" at them again. If the fans are stretching up to the light and seem happy and healthy, it was a good change. If the fans are drooping and dont seem as healthy and happy as before, it likely was a bad change. Good luck
