Here's what happens. As water evaporates, it leaves behind what's dissolved in it. For example, if half the water evaporates, you will have twice the minerals, metals and nutrients as before. This happens gradually, and fish don't notice it. They adjust as the water slowly changes. But, when it rains, if the chemistry difference between remaining pond water and fresh rainwater is too great, is distresses the fish and they can die.
There are several things you can do. One is to aerate. Another is to scale back on the feeding, to decrease nutrients levels. (Remember, for every pound of feed, you get half a pound of fish and half a pound of fish waste.) That waste makes its way into the pond and your water has to deal with it. That's one reason aeration can help. Another thing is to have the water analyzed for pH, hardness, alkalinity, metals and minerals. Depending on the findings, there may be something you can do to adjust the water chemistry to prepare for big volumes of rainwater. Another thing you can do is remove a foot or two of the bottom water. That water is often the "heaviest" and has a greater load of nutrients and fish waste. By removing some of the "bad" water, you can set the stage for an influx of fresh water and minimizing stressors to your fish. But, during a drought, this is a tough gamble. Heavy feeding seems to be a problem during periods of low rainfall. It's about water quality. One fish dying is a solitary event, but several fish dying is a trend normally related to water quality declining.
