After oxygen, the next most common cause of fish death in ponds is poor water chemistry. This happens when proper pond management is ignored.
Of course, natural factors contribute to the chemical composition of any water body.
In regions where soils tend to be acidic and limestone is commonly used on pastureland, water draining into ponds is acidic and very low in dissolved minerals.
Under these conditions, most fish will survive (though grow slowly) but will die when stressed by other environmental conditions.
The problem is most common in the winter months, especially following rainy periods. The usual result is a few dead fish every day. These fish will typically be of several species and sizes. The best solution is to prevent this problem by having your water checked for pH, hardness (calcium and magnesium) and alkalinity. In ponds with low pH and minerals, the solution is to apply 1 to 2 tons of crushed limestone (ag-lime like they put on pastures, not hydrated, quick or slaked lime) per surface acre spreading it as widely as possible in the pond.
This usually results in a quick end to fish losses. The treatment may last from one to several years depending on the water and rainfall. Once this problem is identified in a pond, an annual water check would be a good idea as a preventative pond management method. You are likely to have pH and mineral problems in your pond if:
You are in a region with acidic soils. Contact your local Extension agent for advice. Also be suspicious if there are a lot of pine trees around.
Your pond water is very clear or tea-colored, a possible sign of acidity.
Your pH is below 7 and your alkalinity and hardness are below 25 ppm.
Fish die in cool weather following rain.
The dead fish are several species and sizes.