It must be of common knowledge that all living animal even those in a pond need oxygen to live. If the concentration of oxygen in water is low, your fish will pass away in addition to all the plants, great algae and the valuable germs. A pond aeration can be used to fix such a trouble and this is clarified below.
The simplicity of how oxygen reaches the water in the pond will identify the aeration of the pond. A pond is likely to have imbalance of oxygen if the amount of fish is greater or if you commonly feed them. Such a situation might also occur if bad algae are enabled to form in large volumes in the water. Both scenarios result in shortage of oxygen in the pond that is damaging to the life and developing plants and animals in the pond.
In natural situations, the water aerates itself without intervention. This procedure stops to work when human disturbance disrupts this balance of nature. The obstruction could come in terms of overpopulating the pond, or feeding the fish too much too frequently. As constantly it is people who cause these disturbances, so it takes humans to rectify the circumstance as the natural process could not go back the damage done.
It is most likely that you have actually come to see that it is mainly on an extremely hot day throughout summertime when winds are missing that the fish in the pond die especially if you do not have a solar aerator. This is an outcome of inadequate oxygen in the water. There is also a maximum volume of water per minute that each pump is able to handle thus choosing the right one is essential. This is because they consume co2 and give out oxygen enhancing its level in the water. This means that such ponds should have actually less fish presented to them or be larger in size. If the problem can not be fixed by all these measures, purchasing an air pump need to be the next action.