PH problems in reservoir

Hello
I`ve had 3 grows in soil, but switched to coco/hydroton this grow in my autopot system.
I`m using a gravity fed reservoir with a powerhead as circulation.
My problem is that when i PH my water using GHE PH down to 5.5, after less then 24 hours the water in my reservoir is at 7.5+
Tap water is coming out at 7.2, and i dont know the exect hardiness, but its middle to high.
My thoughts about using autopots for a handsoff aproach is failing fast, its like taking care of a tamagotchi atm with all the PH readings/dosing.
Any help would be appreciated.

Yep, that happens. Not sure what the scientific reason behind it is. You just need to dose it a again...after a second ph downing, it will stabilize. It takes much less to get it down a second time.

Thanks
Yes, I`ve noticed that too.
Coming from the aquarium world we would add crushed coral as a buffer to keep the PH at 7+, could something equivalent be possible in our reservoir too keep the PH down ?
Peat moss works as a ph- buffer, but you have to replace it often and it releases tannins.