Sick Koi Found as Spring Arrives

I have a 20,000 gallon pond with 60+ Koi. The pond is spring fed with a bead filter and a three chamber biological filter. This is only my second winter in the house. Last year all was well. This year there seems to be a lot of string algae that I plan to muck and filter out. This year the pond did freeze over for a few weeks. Last year this didn't happen.

The Koi stay near the bottom of the pond most of the winter, so it's hard to check on their status. To my surprise when they surfaced for their first feeding of cheerios, I saw that two of my large Koi had a form of fin rot. About three others seem to show signs of a milder condition. All the others seem just fine.

The water quality is good, although I seem to have more algae then usual. All tests are within the normal parameters for my pond.

What should I do?

See photos of two of the fish below: