The drinking water must have 0 buffer to fall so rapidly to such a.low level. I don't want to sound like an expert - I'm still learning lots about water chemistry, but I do know that 5.6 pH is far from ideal for the development of biofiltration. My research says 9.0 pH is optimal for the growth of nitrifying /denitrfying bacteria. Of course we don't want THAT high ph in our tanks for the fish, but just goes to show how bad low pH is for your cycle.
I will be following this thread closely to learn from this highly unusual situation. Thank you for posting your results and please keep updating.
Best wishes for beautiful and healthy aquarium!
"Be the change you want to see in the world."