A garden pond filter is essential for maintaining a healthy pond if you plan on creating a pond with fish in it. Fish put toxic byproducts into the water that the filter will need to remove to keep the fish and plants healthy. However, there are also other reasons to add a filter to your garden pond. More on that below.
There are two types of filters for garden ponds: biological filters and mechanical filters. Biological filters are needed when you have fish in your pond. They remove the fish waste and chemicals like ammonia from the water. Mechanical filters just remove debris from the water. While some of this could be solid waste from fish, it could also be dust, soil or algae. When the water is returned to the pond it has been cleaned of all particulate matter.
A mechanical garden pond filter can also be useful for other things. Because it removes small particles from the water, it can prevent the water from becoming stagnant, allowing the same water to be re-circulated for a long time. The mechanical filter also can perform some non-filtering tasks like providing the constant water flow for a water feature, or just providing the pleasant sound of gently running water.
Obviously, the size of your pond is the primary consideration when deciding what you need. It does not hurt to have a filter that is a little too big for your pond, but you should never run the risk of getting a filter that is too small. Also keep in mind whether you want a water feature, since this may determine the filter size because of the amount of water flow you want.
There are several type of garden pond filters. The simplest is probably the internal filter, which consists of foam blocks that sit in your pond. While simple to install it as a few inherit problems. It eventual gets blocked by the accumulated solid waste, and it can harbor infestations of anaerobic bacteria.
One type of filter is completely sealed, which is called a pressurized filter. They return the water to the pond through a plastic return tube or water feature. Because they can be buried, you can make a pond look completely natural, with no mechanical devices in sight. However, they need regular maintenance and tend to become blocked relatively quickly. Because of this they are better suited for small ponds.
For medium to large ponds (up to 5,000 gallons), you should consider using skimmer filters. These are typically installed immediately adjacent to the pond, accessible through the pond wall.
External filters are very efficient, but somewhat expensive to create. They are placed in a location outside the pond so they can be easily maintained and make it convenient to test water quality. If you want to install an external garden pond filter, you have a couple of varieties to choose from.
The easiest to install external filter is the pump-fed single-chamber filter, which explains why it is so popular. With capacities that can handle up to a 2,000 gallon pond, they are sold at most commercial pond suppliers.
If you are serious about creating a natural ecosystem in your pond then you want to look into building a gravity-fed multi-chamber filter. The main drawback for this type is that they can be quite expensive. A gravity-fed garden pond filter is built beside the pond at the same level as the pond so that the water level in each is the same. As water is drawn from one end of the filter, gravity pulls the pond water into the filter. There it passes through multiple chambers separated by blocks of filtering media. The water then exits the filter through the transfer ports which causes more water to be drawn into the filter intake.
Regardless of how ambitious your plans, picking the right garden pond filter is key to maintaining a healthy pond. While this was mostly concerned with mechanical filtration, if you plan on introducing fish like Koi into your pond, you will need to pay equal attention to biological filtration.
