First off, why are your nitrates bottomed out??? Is that tank cycled? It better be if your going to add in Discus or that could be a costly ending.
Are you injecting CO2? Whats you kH? If your kH is really low, the buffering capacity of the water may be volatile, so the slightest natural buffer could be bottoming out you pH (which is what I am suspecting). You say pH is 8.8 out of the tap??? Check that again.
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