I don't have any knowledge regarding the low ph being responsible (or not responsible) for leaching po4 from the rocks. I am assuming that you are using dry rocks? Possibly that's enough to leach po4 rather than the ph. The only time ph would cause any significant issue is when it starts to approach the neutral level (7.0) which is not very easy to do unless you have a sudden decrease in oxygen level or keep the tank in an airtight area. To me, 8.4 and balance would be a very poor thing to add to the tank due to the fact that it gives a temporary boost and then will go back to the "equilibrium" that you tank maintains; the primary factor (I think) would be the ventilation in the room.
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You wouldn't want to see my tank. I don't use fancy equipment and I am a noob 
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