I dont think they prefer high pH. I think nitrifying bacteria can not tolerate low low pH.
Learned that when pH crashes in well established tank, tend to see
NH3,
NO2 creeping up.
Lots of people make/made mistakes when this occur by just raising ph, even gradual increase, w/o checking for NH3, NO2. As More ammonia will shift from nion toxic form (ammonium ion; NH4+ ) to toxic Ammonia (NH3).
Should always control NH3, NO2 via water changes and controlled feeding (skip feeding)
before manually/chemically raise pH. When water changes are done, your new/tap water is higher in pH than tank water. Thus w/ water changes, both NH3, NO2 will be reduced and at the same time pH may be up to point where bacteria will/can become active to perform their duty. Once NH3, NO2, is under control, should add chemical/Crush Coral to raise the pH to desired level.
It seems your water has very low buffering ability. If deired pH is @ 7, try adding Neutral Regulator(
buffer formulated to keep pH @7) ONLY when you brought pH to 7 w/ chemical(sodium Bicarb; pure baking soda w/o any additives)or with lots of water changes.
If desired pH is 6.5, I believe buffer for that pH is on the market)
Remember nothing last forever since these buffers are constanty being exhausted battling pH drop. Need to monitor regularly to avoid pH crash.
Crushed Coral (will raise and buffer but water becomes hard)) work as well but be careful with fish requiring soft water (discus, etc)
Similar Aquarium Fish Forum Threads 
Thanks, Nate
) added a small handful of crushed coral in a media bag into the filter. I rinsed and rinsed and rinsed some more then dipped into stresszyme treated water for awhile to detox the stuff. I added it to my filter for about 3 hours while I was gone this morning and my PH from the tank (which tested at 6 or less several times) is now at 6.8. I have removed (after I read the post above!) the coral for the time being. 
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