Probably I shouldn;t have done it and may be very sorry, but----
This morning I went out and bought six water hyacinth. They will add some
shade and reduce the oxygen and maybe, just maybe, I will be free of all
that algae.

During the day, the water hyacinth will add, not reduce, oxygen to your
pond. At night, they may use it up, particularly if your pond is over
populated with it. One way to ameliorate this would be to make sure that
the water is properly circulated at night. Bottom to top, destratified
somehow, perhaps run through a waterfall if that's not too noisy.

If your pond is small, you won't have a problem with the WH, as long as
you cull them once in when. Actually, this is good, because the WH will
concentrate wastes in a form you can easily access. Toss it in your
compost heap.

If your pond is a large natural one, you may be very, very sorry.

Here's what can happen:

http://tncweeds.ucdavis.edu/photos/eiccr04.jpg

C//