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That's weird. TDS never goes down, it only ever goes up unless you did a water change with pure RO water, or perhaps a misreading from the TDS meter. Try testing it again. Check your TDS pen against Pure RO water. It should read close to zero. Check the TDS meter against tap water, or something you kinda know what the reading should be to make sure that the TDS meter is still working as expected. To loose calibration by 100+ is quite a lot. You can try brushing the TDS meter's probe with a soft brush under tap water to make sure there is nothing effecting the reading.