I would suggest you do not drain the pond totally again. This is a
quick fix but a temporary one as you will have wiped out a lot of the
biological filtration that was in the pond. Those great heterotrophic
bacteria are not limited to just our filters, they grow on the surface
of the liner, the rocks, ect.

You could get a UV sterilizer or clarifier for free floating algae.
Many people use barley pellets, or straw or hydrogen peroxide to
control string algae. It is also recommended that the pond has a 70%
surface coverage of plants, but I did read a research article that
showed in one study that there was very little difference in the rate
of algae growth in the shade and direct sunlight. Very interesting.
This article also stated that the nitrifying bacteria in an established
pond secretes an enzyme that interferes with the growth of algae.
Another reason to do partial water changes vs draining a pond.

Since I have been using barley hay the last two years I have not had
any string algae. When it breaks down it releases hydrogen peroxide.


Hope this helps- I am sure others will chime in with their
recommendations as well. Happy ponding.


--
Koitoy