If your pond is that old, it's highly unlikely your water is soft, even if you get a lot of rain and flow-through. Still, a water analysis would be useful to see where you stand.

Do you have catfish and/or carp in the pond? I bet something is stirring it up. If that's the case, no amount of lime will eliminate the muddiness.

Is the low visibility from mud, plankton bloom, or both?

A simple jar test will quickly identify mechanical turbidity. Take a mason jar full of pond water, and let it sit. If it settles and clears in a day or so, something has been stirring up your water. Time to catch some catfish!

If the water does not clear up, and is mud color, then lime may help settle it over time.

If the water does not clear up and is greenish in color, it's probably a heavy plankton bloom, which isn't entirely bad.

Shrimp