Algae in ponds?

[-Asianguy-]

Are you talking about the filamentous green algae? American cats aremainly carnivores and won't eat much of it.
Here are 2 types of USA fish that eat it reliably and live in cool-cold waters, both pics are from the Native Fish Conservatory. They are the flannelmouth sucker (Catostomus latipinnis) 22", and the highfin carpsucker (Carpiodes velifer) 20", I don't know where you would find them there.
Another possibility, especially if you can find older ones, might be easier to find. That is the Chinese high-finned banded shark (Myxocyprinus asiaticus) 40". People buy them when they are small, not realizing how big they get, or that they change shape and color as they grow, so you can often find them cheaper atthe larger sizes than when small. They do eat pellets and worms but they mainly eat algae, a lot of algae. Here are two pictures, the young fish pic is by Johnny Jensen, the adults are in a pic from the Science Academy of Chongging City.

what is the second picture? the shiny silver one?

 

[Muske]

That algae looks like string or hariy algae. It is a natural process that will happen at different times of the year (usually spring and fall by me) As for fish to eat it I don't know of any off hand, not to say that their isn't any. I use a small plastic rake. The ones you see for kids. Try to find one with a long handle. When it floats you can just scoop up the mats that form. Another good way of collecting it, is to use the small rake like a fork with pasta. Twirl the rake in a section of the algae like you are trying to get speghetii on a fork. It doesn't get all of it but it is easy.