You said your pH is at 7.8 (which is almost neutral) and then you want to neutralize your pH? This is confusing. So you need your system to be running at 5 or less. That would be acidic so you would need to decrease the pH (which they make pH decreasers and sell them at most pet stores). I'm not sure of a more natural way, although I would recommend you get a hold of the water quality first before messing with the pH.
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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."