Kaze...Your problem sounds very similar to the problem i had with my pond several years ago. Like yours, my pond is shaded for part of the day (more so in winter) and partly exposed to the sun for several hours. Summertime it is intense sun and some extremely high temperatures (high 40 degree plus celcius). I have been through the algae problem big time...miles of string algae and the green slimey stuff. Up to my waste in water. hauling out yards of algae and crap every week...expensive chemical treatment and two ultra violet filters.It was a continual nightmare.
These days...not a problem, no chemical,no ultra violet filters and no algae.
There is a product called an i-tronic that is made by "Oasis" and "Velda" I installed one of these on advice from the aquarium store that i deal with in Perth (Aquotix) and now we have crystal clear water all year round. My pond has a heavy fish population and we have had no problem with any of the fish since the installation of the I-tronic. It has also taken care of a massive snail problem that we had at the time but has not affected the yabbies that breed in the pond.It has not affected the breeding of the guppies, rainbows, bristlenose and cichlids that are prolific in the pond.Check it out on the net and have a look at my pond ( backyard pond pics) in this forum a couple of weeks ago.I would be interested in your thoughts....Regards...Ray.