Stain in oak table top

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I bought an unfinished oak table and finished it
with a store-recommended finish to get an oil-like
matte finish. When just a small amount of tap water
spilled on it when watering potted plants (and was
quickly wiped off), it developed a black stain
(fungus?). This has happened several times now on
this table, which I keep covered until I can refinish
it with a lacquer this time!

Does anyone know what has caused the stain, and will
sanding remove this stain?

Thanks.