This is very normal. It's abnormal when it doesn't happen. Simple forum type answer is to add a UV filter. Long answer It's not specifically related to the goldfish. The fish do produce ammonia which algae uses to grow, but and this is a very important but, do not jump to the conclusion like virtually everyone does that the algae wouldn't grow if the fish are removed. It would probably grow less fast, but you'd still have the exact same problem. It's the same with sunlight. Algae grows slower in shade but still grows. To most people it doesn't really matter if there is 10,20% less algae, it's still too much. The other important thing to understand is in ponds we break algae into 2 general classes. 1. Green Water Algae, aka Micro Algae Micro just means small. This type turns water green. Meaning if you put pond water into clear water glass and hold it up to the sun the water has a green tint. Generally pond owners will say something like "I can see the bottom" or "I can't see the bottom very well" or "I can't see my hand when it's 6" deep" to describe how green the water is. These green water algae is a single cell algae that is so small it stays suspended in the water. Although these cells will clump together into larger balls and then sink eventually, but new cells will replace those. Also called suspended algae. 2. String Algae, aka Macro Algae Macro just means big. String algae is an unfortunate term because it's only one type of macro algae, but string algae is the term most often used in forums. There are many species of these kinds of algae. These are more like plants, you can touch them, pull it out, etc. Your description, which is common, covers both kinds. It's important to know which kind because the remedies are very different. The string algae type stuff generally grows on the pond walls, rocks, pipes. Some people seeing this say my pond is green even though the water is clear because they see the green stuff growing on the pond walls. You said It happens both in the pond itself, hosing and the waterfall. That describes macro algae which grows on things. But you also say can no longer see the fish which sounds like green water algae (micro algae). Micro algae does clump and sink and so the pond bottom, hoses etc, can get covered in a kind of green dust. To most people that could appear to be algae growing on those objects. You also said green scum. This is a term we use to describe a green film floating on the water surface. It can be very dense, almost like green paint floating on the water. This is a less common algae but could lead you to say you can no longer see the fish. So it's kind of important to narrow your description. Many people don't like algae and so like to use terms like "scum" just as a way to insult the algae. But this type of thing will only generate bad advice and make your life much more complex. Besides, algae don't read forums so they won't be insulted at all. But if your algae actually is scum then that's an important clue. The time frame you're describing also sounds like micro algae. New water, couple of weeks (2-4) and the water turns green. The most effective fix for green water (micro) algae is a UV filter. These are pretty cheap to buy and run and are 100% effective in removing green water (micro) algae in 3-7 days. It is so effective that the test for knowing if you have it sized and installed correctly is the color of the water. If there is any green tint at all it isn't sized or installed correctly. They're not hard to install, but unfortunately many people don't seem to like any kind of detail. They plug them in and if it doesn't work give up and start trying the many "fixes" that don't work. Such is the world. After the water is cleared you will very likely get a few kinds of macro algae. Interesting when this happens you can most likely turn off the UV and you may never need it again except may in the spring depending on your climate. Macro algae produces a chemical that kills micro algae. But micro algae also produce a chemical to kill macro algae. A war. The UV filter kills the micro algae and so their chemical goes away allowing macro algae to grow. Macro algae can be controlled by chemicals you buy, but I'm not a fan of those. Lots of issues, but is possible. Much easier to remove macro algae by hand. Like weeding the garden or mowing the lawn. Emptying the water and refilling is the most difficult way to deal with algae. You will often read in forums people saying "you're starting over" because it's widely believed ponds clear on their own given time, but most people don't know why. It's because of the chemicals produced by macro algae. Another effective when to deal with green water algae is to have a constant water drip into the pond. This is tricky in small ponds (100-200 gals) and almost impossible in most pre-form ponds because an overflow is needed. Here's a video that describes the system.

I scrub my birdbaths with a brush and have never heard of putting copper in them to reduce algae. I do know that copper is toxic to fish so I would not use it in my pond though I use it near my fishpond and my frog ponds with no ill effects.

I have a small 250 gal. pre form pond & I too have had success with using peroxide to keep the algae growth down. I use half a quart bottle of that WalMart 3% &dump it near where the water comes out of the Skippy filter (& make sure there are no fish directly where I dump!). I don't want the peroxide to kill the mint I have growing in the Skippy. I read to use this somewhere on this forum & it sure seems to work! I empty the filter in winter & toss the mint but I always have more in the herb bed to use the next year! I put the peroxide after it comes out of the filter as I didn't want to harm the filter/mint system in the Skippy. Have had clear water now for the past two years & only the short green stuff that clings to the pond sides that the fish love to nibble at. Thank you Jill for verifying the use of peroxide!!

suggi- you could crumble some Mosquito Dunks in it- won't hurt the birds. you can buy them at big box or garden stores or online. you will have to keep doing it though, as they eventually dissolve. don't get caught! we have a neighbor like yours and i am actually fearful of his rage! sorry you have one too. min