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why did my fish die after it rained?

• Rain often alters the water chemistry. Rain water is almost distilled and has no buffering capacity or hardness. Sometimes it is even acidic in pH. Adding a lot of rain to a pond will often lower the pH, alkalinity, and hardness. When the alkalinity goes down, the pH changes faster. If the change is drastic enough, the fish can suffer and even die. Test the pH, alkalinity, and hardness of the pond water and compare it to your normal readings to verify if this is the cause. If so, pond chemicals can be added to buffer the water (such as sodium bicarbonate which is baking soda) and/or raise the pH if needed.
• Rain is also low in oxygen. If there is a lot of rain, the oxygen levels in the pond may plummet. Fish can be seen gasping at the surface and hanging around near waterfalls. For this reason, it is a good idea to provide aeration at all times in a pond but especially when it is hot out as warm water holds even less oxygen. A single waterfall may not be enough. You can buy air pumps with an air stone for more oxygen. Unfortunately, when a storm comes through, the electricity often goes out. See this section for more on power outages.
• Rain will flush things in to the pond from surrounding ground. That may include mulch (bring in lignins and tannins and sometimes lowering the pH), dirt (clouding the water), and chemicals. If there are pesticides, herbicides, or fungicides used in the area, those may end up in the pond and kill fish. You can control what is used on your land but not necessarily the neighbor's land. If a neighbor uphill from you uses chemicals, make sure to divert rain water away from the pond. See this section for more on that. If chemicals (natural ones or human made) end up in the pond, add a fresh mesh bag of activated carbon to the filter or in moving water to remove them.