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Day on day comparisons are pointless, for the reasons you mention - weight fluctuates by kilos a day, due to water / hydration if nothing else.
Even week on week comparisons have large margins of error.
That's not to say that you don't need to weigh daily (at the same time, in as similar conditions as you can to miminise confounding factors) if you are to infer any sort of trend with any sort of accuracy.
Just don't infer anything from the daily moves themselves.
@mashton I wouldn't bother with daily weigh-ins as weight can naturally flux.
Once a week is better plus you become less obsessed with it...