indeed, my thoughts would be the 'sand'
unfortunalty most sand is not inert and actually leaches carbonates and calcium into the water which can increase the pH. as the buffering ability of rain water is very low it would only tank a small amount of leaching to raise the pH.
The other option is to head to the local supermarket or hardware store and grab some distilled/demineralised water. test the pH of that then add some of the sand and test evey few hours to see if there is a change. if there is then your sand is not inert and will be the issue. Ceramic noodles are 99% of the time not the issue.
either of these test will help out. if it is the sand then the best replacement (if you still like the sand) is silica sand, the type used in pool filters. This sand it totally inert and cheap as too!
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