I still wouldn't add anything that has potential to raise GH in this case, that is already pretty high. I wont recommend limestone and crushed corals. because they will also raise your GH.
I would use a commercial KH booster to raise the alkalinity only and slowly until the PH moves a little over 7. Then maintain it trough water changes.
I would use a commercial KH booster to raise the alkalinity only and slowly until the PH moves a little over 7. Then maintain it trough water changes.
