If you are using RO then you should be able to get the pH stable.
If you are using tap water,
Then these are the rules for tapwater,
or water over 50ppm and/or initial pH of near 8.
After you add nutes to your water,
the pH should be close to 5.6.
If your water is very hard, over 200ppm,
then you will need to add pH down
(Phosphoric acid, or Sulphuric acid)
to achieve 5.6.
In a few hours,
as the buffers absorbs the acids,
the pH will move up,
back to near the original pH of the water.
So, you will need to continue to add pH down,
at least twice a day,
to re-establish the pH back at 5.6
After doing this for a few days,
the pH will become stable for a few hours,
maybe even for a day or so,
and then the pH will fall below 5.6 without adding any pH down.
When the pH falls without you adding pH down,
you have met the pH rule.
This is when it is time to change the nutes,
even if you have not met the Add Back Rule
For some nutes,
it is best if you always let the pH move from low to high,
and then you adjust it back down,
while other nutes will react in the opposite way,
depending on whether the mfg decided to buffer the formula or not.
Try to never allow the pH lower than 5.2 or higher than 6.2,
ideally you will maintain 5.6,
but in reality you will probably drift between 5.2 and 6.0
The time that this sequence takes will vary with the plant size,
the amount of water the plant is consuming/transpiring,
and the concentrtation of the nute solution (ppm/nute load).
If you are growing with tapwater,
the pH rule is the best method to determine when to change the nutes.
It can be as long as 10 days, and as short as 4 days.
If you must use tapwater,
It is highly recommended to use a Hard Water nutrient.
Your rez volume, the phase of your grow, and the type of nutes you have
will determine how often you need to adjust the pH.
Your job is to maintain the pH,
and change out the nutes at the appropriate time.
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