Merdav,
You can try a lot of fixes that we routinely post/read about on this forum, but I think Bill hit the nail on the head. Your pond is an immense nutrient sink particularly when you consider that your pond may be 100 years old!! That's a long time for grass clippings, leaves, fertilizers, fish/turtle waste, etc. to accumulate. It probably has been dredged some time in the past but not necessarily. If you begin trying algaecide treatments and find that you are continually having to perform them, then the real fix will be to drain and remove sludge. Given the relatively small nature of the pond, you might even consider draining and installing a high quality liner. I only suggest this because you can cover-up decades worth of organic build-up and because you desire more of a water-garden/aquarium type of setting. Just food for thought. By the way, you have a pretty cool setting. Good luck.
