Oooooookay. Lots of really interesting "info" on this thread. Let's just address a few of the biggest misconceptions:
1. Sun does NOT cause algae. Algae is nature's way of creating balance - too many nutrients, not enough plants, algae to the rescue. Sunshine makes it nice and green, and yes if you COMPLETELY COVER the pond it will die... but then, so would any other green living thing.
2. Emptying and refilling a pond repeatedly is exactly what you DON'T want to do. Every time you do that, you start alllllll over with a pond that now needs to re-seek balance. (And yes I do realize I just posted about completely emptying and cleaning my pond - but that's the first time in 14 years folks. And I do expect to get an algae bloom at any minute.) You need to let your pond mature - that "aged" pond water is like a fine wine. The older it gets, the better it gets.
3. UV works to kill one type of algae - single cell floating algae. In order for it to be eliminated it needs to pass by the UV light somehow in the filtration process. UV does nothing for string algae or carpet algae or even that nice gloopy floating algae that we've never really named. And UV also kills every other single celled creature in your pond.
4. Chemicals WILL kill algae, but what else are you killing? AND when you kill the algae in your pond, you're left with a pond full of dead algae. Guess what algae loves more than anything? Rotting organic material in your pond. Potassium permangate? Gee. That's like using a flame thrower on a mosquito. You're medicating your whole pond population at that point. Not a good idea.
5. Barley straw is an old pond idea which is fine to try, but remember it works by rotting in your pond and releasing hydrogen peroxide. Why not just go right to an easier route and buy some H2O2 at the drug store? Or buy (or make) barley straw extract.
NO POND WITH LIVING PLANTS OR ANIMALS SHOUDL BE ALGAE FREE. If you have a water FEATURE and want to keep it free from algae, go ahead. Use bleach if you want. Just be aware of the creatures that might avail themselves of the water - birds, bees, butterflies, chipmunks, squirrels, etc all use my water features as a spot to get a drink - and keep the level low enough to not be killing anyone.
If you want to control the NUISANCE algae in your pond (mainly string algae) address the source. Maybe it's spring and your plants haven't started growing yet - then just wait. But if it's full on summer and your established (more than three years old) pond is still teeming with string algae, you need to recognize that you have too many nutrients in your pond. You fix that by:
1. Add more plants
2. Reduce your fish load
3. Feed your fish less, or not at all
4. Remove rotting organic material from the pond - and stop adding more organic material by killing off the algae with chemicals and UV.
This forum is a great source of information for current pond owners as well as people who will come here years from now seeking advice. It's just very hard to read misinformation, knowing others will keep repeating the same because they read it here.