until a few days ago, i had a serious problem with algae in my pond: almost the whole surface was covered in green goo.
i tried scooping it off, but it just didn’t go away.
even though there is no (apparent) source of fertilizer, the causes were most probably eutrophic conditions (too many nutrients available) – very common for small water bodies.

then, i transferred one bucket of water from a nearby lake/big pond with a functioning ecosystem.
within one week, all the algae were gone, and i have a perfectly clear pond. you can watch the dragonfly-larvae crawling around on the ground!

it’s amazing what a bunch of bacteria can do in such a short timespan.

(i should have taken before- and after photos, but i forgot…)