Parrot's feather not flourishing in wine barrel fish pond

I have been maintaining a wine barrel fish pond for about a year now.
In about April I took out some parrot's feather that wasn't doing so
well and replaced it with a new hunk of the stuff. About a month
later I also put in a couple of water hyacinths. Goldfish have been
living in this pond for about a year, on average about two at any
given time. One individual goldfish has flourished for twelve months.
(Others have been picked off by birds and cats and been replaced.) So
I think it's a generally healthy environment.

My parrot's feather isn't doing very well. There are lots of tendrils
under water and just a handful of tiny shoots above the water's
surface. At the same time, since I have added these plants, and
increased the shading, the algae has come under control. For a while
it seemed the algae were taking over. That's positive, I think.

This is a hot, dry climate during the summer: from the 80s to 100+
during the summer. Inland Northern California. The pond is in the
shade most of the day, though it has a few hours of full or partial
illumination.

I thought parrot's feather was an uncontrollable weed. Any ideas on
why I can't get it to flourish?

Thanks.