I have always, happily, kept Duckweed as a treat for Goldfish but I have, either been fortunate or never noticed the various Wollfias. Once we (myself and friends) did not appreciate Hornwort but I have since been told that UV gets rid of it. The plant was great in a tank but any small piece regrew very fast and became a pain.
I do not have any sure methods of getting rid of Wollfia (try the UV, if you can) but it's green and growing so just net it out and throw it on the Lawn or flower beds while you count your blessings for the growth and the fact that it's not Algae. Otherwise get a few Goldfish and keep feeding them the unwanted green stuff. Also do lots of water changes. Any natural treatment such as the net is superior to any chemical even though it may seem as if the chemical is easier.
I do not have any sure methods of getting rid of Wollfia (try the UV, if you can) but it's green and growing so just net it out and throw it on the Lawn or flower beds while you count your blessings for the growth and the fact that it's not Algae. Otherwise get a few Goldfish and keep feeding them the unwanted green stuff. Also do lots of water changes. Any natural treatment such as the net is superior to any chemical even though it may seem as if the chemical is easier.