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In RO/DI or distilled water, pH is not really meaningful because a very tiny shift in ions or CO2 can appear to cause a BIG shift in pH. The salt ions you're adding completely overwhelm the tiny bit of acidity or alkalinity in the RO/DI water. Even if the RO/DI pH measured 4.0, the pH would still go to whatever pH the salt mix is designed to achieve.
 

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I am going to guess your RO/DI is indoors and with the weather as it is I am also going to guess you haven't had your windows open much either. Gas heat or fire in the fireplace?

The reason I ask is because CO2 buildup indoors will acidify pure freshwater (pH 7.0). It is common in the winter for this to happen. Actually in any enclosed space without consistent ventilation. Many of us have been to a frag swap and seen the pH meter in an aquarium drop throughout the day.

Try taking a glass of water and putting it outside for about 15 minutes and see if it levels back out at 7.0.
 
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