So for 10 years now I've had a small pond in zone 6 that has been self sustained. It's a small 6 foot wide butterfly-shaped pond. It has several healthy goldfish that survive over winter since the beginning under the ice. A couple as long as 6 inches. Some years we have tadpoles. We have several plants. No filtration system, we just have a gutter downspout go into the pond so when it rains there is some fresh water getting in there. It all overflows when we get a lot of rain, so it flushes out some surface debris.

So it has a layer of fuzzy algae along the bottom of the pond, which I don't really mind. But the water is and has always been a little murky (visibility is only the first 6 -8 inches at the top of the water). I would like to address the murkiness. I love having this happy self-sustaining pond, and so I don't want a filter. Is there some type of treatment I can add to it once in a while to clean up the murkiness?