How about putting some barley straw in it? This seems to contain a
natural algicide and is recommended for controling algae in ponds. I
have tried it for this and it seems quite effective.

Because we live in inner city Leeds where barley doesn't grow, all our
barley straw has been used for skeps and it's a long wait until harvest ...

:-)

But I really don't mind the algae, I only asked if it was deleterious for
watering in the gh.

Mary
--
Larry Stoter