If you've chosen to build a garden pond that doesn't include any fish, you obviously don't have to worry about removing ammonia and other harmful fish byproducts from the water.
However, it's still important to use a pond filter to aid in several other important processes.
Pond filters can provide mechanical filtration, biological filtration or both.
Biological filtration is most commonly used for removing fish waste, so without the presence of fish, this process is unnecessary.
Mechanical filtration, however, is the process by which pond filters pull in dirty water, remove debris from it, and re-circulate it back into your pond once it's clean.
Clean water is directly correlated with healthy water, so preventing dead plant life and other detritus from accumulating is a big part of helping your pond thrive.
In addition to cleaning your pond water, mechanical filters are also responsible for:

• Preventing water from becoming stagnant
• Circulating the same water for long periods of time
• Providing the soothing sound of running water
• Aiding in the creation of water features such as waterfalls and streams

There are many mechanical pond filters to choose from, but the main factors to consider are the size of your pond and whether or not you want to create any water features.
Small ponds don't need huge filters, but you never want a filter that's too small to get the job done.
If you want to add eye-catching water features to your pond, you should use a mechanical filter that has spillways and bulkheads that can turn into waterfalls, streams and more.
No matter what type of pond you're creating, filtration is an essential part of maintaining a clean and healthy aquatic environment.