Atlantis

If adding malt vinegar (acetic acid) does not reduce your pH, then either the test kit reagent chemistry is faulty or the water is being bufferd by carbonates or bicarbonates. Again my recommendation as a water chemist is to get a decent pH meter (LaMotte Tracer or equiv) and calibrate with pH buffer solution then look for pH variations as you slowly introduce dechlorinated tap water (for fresh water aquaria)

Most test kit reagents have a shelf life of between 18-24mths. Both pH meters and TDS meters provide a more accurate and reliable method of analytical assessment of water chemistry. Like most things in this world you get what you pay for. Having used pH meters and TDS meters in both "domestic and professional aquaculture use, the cheaper electronic meters tend to fail from corrosion or worse lose their calibration giving you false confidence. Calibration solutions and pH buffers are readily available. My advice is to use a "middle market" pH stick meter such as the LaMotte Tracer or equivalent - not a cheapie but not full laboratory grade.