In addition to the pH, you need to know what the alkalinity is. It is the alkalinity that "buffers" the water from dramatic pH swings. Think of it this way; a low buffered water can be neutral pH like demineralized water. A single drop of acid will drop pH into the acid range. Likewise a single drop of caustic would raise pH well into the caustic range of pH. Alkalinity concentration buffers the water such that more acid or more caustic solution is required to have that same impact on pH. Alkalinity is the combination of carbonates, bicarbonates and hydroxides in the water.
Your well water may be high in calcium and magnesium hardness, but low in alkalinity, or the converse. Surface waters usually are lower in alkalinity than ground water, so have lower buffering. In east Texas, pine forest runoff lowers pH of ponds, thus the need to add lime to add buffering and raise pH. As the water gets lower in pH, there is more negative ion concentration, so more tendency for water to stay cloudy.
As others have mentioned, I would stay away from hydrated lime, unless you add sparingly, with good mixing, to avoid concentrated areas of high pH. Getting the water above 9.0 pH is detrimental to the fish. Ag lime has a lower dissolution rate, so the time required to fully dissolve usually tempers the impact on immediate pH change. I would also recommend a jar test taking several samples of pond water, divide into several jars, and add drop wise with mixing, a solution of alum. When you find the amount that it takes to clear the water in an hour or so after adding alum, that is the amount you will need to clear your pond. Test that sample pH to make sure it has not dropped below 6.0 pH. If it has, then you will need to bolster the alkalinity 1st before adding the alum to the pond. Which ever source of lime you choose, you will need to do the jar tests again, to determine the rate of alum to add, since you changed the water chemistry from the lime addition.
Also keep in mind that a jar test is a static model under "lab" conditions. You will not be able to get the same level of mixing, distribution, and the water chemistry is not uniform across the pond. Looking at water in a jar across a few inches also looks clearer than looking through feet of water in a pond For these reasons, the amount shown by the jar test may not work exactly the same in the pond. More alum may be needed to get the results shown in the jar. Dealing with changing the water chemistry in a pond can be more art than science.
One last thing, you can effectively clear the water, only to have it cloud again if you do not control erosion. Too many discount the impact of wind erosion on shore line as a source of silt pick up in the water. That is why many new ponds are cloudy and clear with age. They develop a shoreline vegetation cover.
Good luck.
Mike
