Doesn't matter what environmental changes you make, you will still need
to kill what's there.
a series of water changes can likely succeed, so can the blackout I
suggest which addresses the plant's needs with the KNO3, CO2 etc.
Carbon from the CO2 is 40%+ of the dry weight of the plants.
K and N from NO3 are a good lion's share of the rest.

Blackout is simple and takes 3 days, BGa will come back if you do not
add the KNO3.
KNO3 you can get at home dept for 5$ for a year's supply. Most folks
have a 1/4 teaspoon.

Pretty simple.

Regards,
Tom Barr