Buying a pond pump for your fish pond means you are investing in the future well-being of your pond and your fish.
The pond pump is primarily to keep your fish alive by circulating the dissolved oxygen in your pond water through to the bacteria that exist in the pond's filter.
The bacteria ensure that it is possible to keep fish in an artificial enviroment. As a result of providing these bacteria with the oxygen that they need the fish and plants in the pond will also thrive.