I had that same problem with my hex tank when I started it. I had a ph around 8.0 and it wouldn't come down for a long time. Since then I have planted it and added fish. The ph went to around 7.6 or 7.8. and the water was still very hard. From the start it tested about 16 or 18 GH and KH. The water in my area has that problem and I am aware of it. Well, I did water changes with water from my purifier and the hardness started to drop. Then I added CO2 with a DIY system. Now it is checking GH6, KH6, PH7.4. I believe it takes a while to bring down the ph having to first deal with the hardness issue. We'll see if my next checks bear this out.
Oh, I forgot... my plants are growing fast and the fish seem quite happy.
Eheim pimp #128, 55gal 13x20x48, Eheim Pro 2227, CO2 injected, into the filter breather tube,4x 55w Pcs , 12 hr light cycle (sunrise/sunset simulated), Milwaukee SMS 122, faulty bubble counter replaced with M3 bubble counter..... moderately Planted, Plantex CSM+B and ferts
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