probably
Elodea canadensis (Canadian pondweed) then - one of the commonest water plants sold, and one of the worst offenders, I'm afraid. Don't let get into your new pond, not even a small fragment
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If I were you, I'd get your new pond built, then get rid of the old one immediately. Dispose of the weed by drying and burning, do not transfer any water from old to new.
T2