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Try removing all the rocks. Then test water the following day.
To add to wilder's comment about expiry dates, you said earlier you were using api testers. They have the date of manufacture disguised in the lot number rather than an expiry date.
I emailed them a couple of years ago to ask and the reply said the lot numbers look somehting like 28A0108. The last four digits are the months and year, in this case 0108 which is Jan 2008. The email went on to give the length of time the various testers lasted from this date. The pH testers last for 3 years.