A water pot full of fish mixed with cheap sere.
Sere is a fish paste. Cheap fish paste smells bad and, if mixed with a great many fish, can ruin the taste of the whole batch. Thus a little bit of a bad thing can ruin a great deal of a good thing.
This is used especially in reference to a person who has only one small bad trait, which spoils his character. It is also used to refer to the bad influence that one person can have on an otherwise good group -a rotten apple in the barrel sort of analogy.
