
Many people believe a pump will keep the water crystal clear. A pump will never keep the water clear. A pump moves water and provides oxygen for the fish if you feed them and the pond is overpopulated. If you have a natural ecosystem there will be no overpopulation and therefore you will not need any pump if you don't wish to have one. A pump will also ensure that you will never breed mosquitoes. I generally suggest that folks have pumps for that very reason. And now that we have reliable solar pumps, lack of electricity is no longer an excuse. Most folks like them because we all like the sound of moving water and waterfalls.
If you have a waterfall, use a pump that moves each hour at least as many gallons as your pond holds. I use a 4300 gph pump, tee the output into two hoses and have great results. Generally, we leave pumps on 24 hours daily, but no pump has to be turned on all the time unless you feed your fish or have koi. You can set your pump on a timer and have it run only when you are outside to enjoy it. With electricity costs rising these days, a timer, available at most big box home improvement stores, might be just the thing to keep those costs down.
And if you live in a climate where you have freezing weather, turn the pump off in the winter. Most pumps do not function well in freezing water. If you are worried about your pond freezing over, throw an empty plastic gallon jug or several in the water during the freezing weather. In the morning, remove the jug and there will be a hole in the ice so gases can move freely during the day. Be sure to put some more jugs in the water when the freezing weather returns at night. There are also deicers available. They keep a hole in the ice day and night.