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I am also inclined to think that it is a measurement error.
Do you get any precipitation or have any type of substrate or rock inside your tank? The only thing I can think of side from measurement error is alk somehow precipitating as calcium carbonate and the redissolving or your substrate/rocks that contains CaCO3 is being dissolved.
But that should not normally happen as alk is calcium carbonate very insoluble in + ~pH 7 and it is very very hard to drop the pH of sea water below that. I doubt it, buy maybe by tight fitting a lid you are causing pH to drop and solubilizing calcium carbonate (if you have any).
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