OK - we seem to have a confusion of terms. "Carpet algae" (not sure if that's a real name or just the type of algae) clings tightly to the interior surfaces of the pond, like a, well... carpet! It's short and dense and hard to remove, but extremely beneficial. There's also string algae or hair algae - the long, flowing, string like stuff that you can easily scoop and remove. Or there's the single cell floating stuff that makes the pond look like pea soup. Which one are you worried about?
In any case - algae isn't a "problem" - it's an indicator. A good amount of algae is a sign of a healthy pond. An overgrowth of algae is a sign that either A. it's a new pond that hasn't matured yet or B. you have too many nutrients in the pond - too many fish, too little water, too much organic material decaying, etc. When you see algae, your pond is giving you a sign of the health or condition of your pond. Don't fear algae - just learn what it means and act accordingly. But in a truly healthy pond you will see lots of good green algae coating every available surface - that's an integral part of your biological filtration.
How big is your pond? How many fish do you have? Any plants? All of those are important keys to pond management.
Before it was around 2000l I would say with 6 gold 3 koi, but there were many mistakes I made with the pond before I expanded it around 2 weeks ago, I didn't do a great job with plants and some soil seemed to have leaked, I assume that created an algae bloom. I didn't feed fish at all during the winter so I can't imagine that fish could have caused it, it was a mild winter and plenty sunlight without shade.
I've not seen any signs of algae yet after expanding it to around 5000l, but the pond should have quite decent shade in about a month after the tree next to the pond grows the leaves so I assume it'll be less likely.
I don't mind algae, but this was like a carpet on the side of the liner. It was a badly executed pond before, now I've added shelves for plants, and the liner is covered with gravel. It was a healthy pond I suppose, since all fish seemed to have survived the winter. Though I don't think I'm overpopulating, I think people probably put way more in a 2000l pond, and now it being 5000l the goldfish and koi are kind of barely noticeable (good for them though, all the space in the world now), I'll maybe add a few more gold or koi but I'm not gonna go crazy.
In any case I learned a good lesson from the previous iteration of the pond.
