You can reuse it, but it doesn't last as long if you mix it with tap water. Mix it with distilled water to last the longest time, and after it is stored or used for a while, measure the pH to verify it is still lower than 3.0. If over 3.0, discard it and mix up fresh.
... but my tapwater is so close to distilled that my 5-gallon buckets full never get cloudy.
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