Not so sure it is fanwort. I don't see the characteristic red stems of fanwort. Does it ever get the white flowers. Flowers sometime during the summer makes it fanwort. Did you crush some of it and smell it to see if it has a musty or garlic odor which means it is Chara normally has longer stem length between "leaf" whorls. Do the individual thin leaflets have smooth or with a few tiny jagged edges?

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