Snails in a tropical fish tank?

My sister has a tropical fish tank and has had snails appear in it! She has never introduced snails to it and has no idea where they have come from! At first she thought they were fish faeces but they are definitely snails altho their shells are transparent. They are multiplying like crazy! Does anyone know... show more My sister has a tropical fish tank and has had snails appear in it! She has never introduced snails to it and has no idea where they have come from! At first she thought they were fish faeces but they are definitely snails altho their shells are transparent. They are multiplying like crazy!

Does anyone know where they could have come from and are they at all dangerous to her fish?
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