I understand that it probaly won't be a popular view, but I still don't think most of what is done for algae control is quite right. I mean, most seem to tackle the problem from the wrong end.
At one time, I had 5 Koi ponds going at the same time. Sometimes, more. What I found was that so long as biofiltration was sufficient, those ponds would go through an initial green water period, followed by a string algae out break, then go clear and stay that way for season after season. Some of those ponds would go through a couple of weeks of green water in the spring, but they seldom did even that after the third year.
Most of the time, the real culprit seems to be fish load vs. biofiltration. I kept rather heavy loads, but also used seriously sized biofilters. A pair of ponds side-by-side, with the same fish load but some differences in the filter systems showed me that I the UV that was placed on one could be, and later was, done without.
Keep in mind that my ponds were in Florida. Plenty of sun and lots of heat. For me, stepping up the filter systems, including the mechanical filter sections, and the old, 10% water changes weekly, along with TIME for the ponds & filter systems to mature, kept both green water and string algae at bay.
Mike S.
Spring Hill, FL