Pond Filters - Why Filtrate?
- Post created by: Matt.E
- 1 Jan 2025
Do I need a filter?
It is necessary to maintain a natural balance in your pond to keep it healthy, so that fish, plants and wildlife thrive and the pond looks clear and pleasant to look at. This post will look at the benefits a filter will provide to your pond.
If you are planning a small wildlife pond with no fish, or one with low fish stocks and high plant numbers (covering around two thirds of the pond area), you may not need a filter, as the natural balance of the pond could be maintained without interference.
However, if you are planning a fish stocked pond, the likelihood is that your pond will need filtration to maintain water quality and enable fish to thrive.
What upsets the natural balance in a pond?
Filters can help maintain a healthy pond eco-system. There are various causes for a pond to be unhealthy and have an upset eco-system:
- Sunlight causes pond algae to grow, starving the pond of oxygen.
- Low flowing water will have a low oxygen content, meaning fish will struggle to breathe.
- Too many fish in an area too small for them will produce lots of waste and uneaten food, and they will not get enough oxygen.
- Decaying plant matter will release methane into the water, which is harmful to fish
What size filter will I need?
The size of the pond, and the volume of water it contains, will govern what size of pond filter you will need. You can work out the water volume by multiplying the average pond length, width and depth (measured in metres), and multiplying this by 1000, to get the water volume in litres. For example:
(Length X Width X Height (Averages in metres)) X 1000 = Volume (Litres)
Filters for Koi and Goldfish?
Filtering a pond that has even a small number of these fish can be rather difficult due to the sheer amount of waste that they produce, It is recommended by most aquatic specialists to over-filter, meaning get a filter 2x bigger for goldfish and 4x bigger for Koi.
Fun Fact!
If you are going for a more natural pond with very minimal livestock and don't want a filter, there is the freshwater pearl mussel in which you can place in any livestock pond or wildlife pond to massively boost the filtration , their extremely hard shell is a great deterrent for all predators.
Biological and mechanical filtration
Traditionally there were two different types of pond filtration systems:
- Mechanical - removes solid waste that clouds the water, by means of a pump that moves the waste through a filter of foam, sand or gravel. Housed inside the pond, it starts working as soon as it is set up and is inexpensive to purchase. These include Pond Skimmers, which by means of suction clean the surface of a pond of leaves and debris.
- Biological - these filters remove harmful particles, from solid waste to ammonia gases, by means of "helpful" bacteria that grow inside the filtration system. Housed outside the pond, this filter can take up to 6 weeks to be up and running, allowing for the helpful bacteria to establish. Once established, though, biological filters are more efficient and eco-friendly.
However, technological advances mean that many filtration systems currently on the market have both types of filtration inbuilt. These type of filters by the likes of Hozelock Pond and Oase benefit from the fact that the mechanical aspect of the filtration system will work straight away in removing debris, while the biological part will create benefits later on, with no extra work required.
UVCs and Pumps
There are two other aspects to a pond filtration system which are often sold in combination with the filter, the Ultra Violet Clarifier (UVC) and the Pond Pump.
- UVCs use ultra violet rays to kill the algae. Green water is pumped into the UVC system where UV rays kill the microscopic single cell algae in the water, making it clump together and enabling it to be collected and removed by the biological filter.
- Pumps will create free flowing water, avoiding the stagnation of non-moving water by introducing oxygen into it. The pump will also move the water into the biological filter, killing the green algae.
Want them all?
Looking to start a new pond or replace an old/broken system? You can start again with a pond kit containing pumps, filters and optionally a UVC.
There are even all-in-one units that can be submersed and do it all from one place, hassle free. For example, the Hozelock Easyclear houses a pump, filter and UVC in one compact unit. These units are also great for pre-established ponds.
Types of filtration systems
There are three main types of filtration systems to choose from, depending on your pond size and requirements: Gravity Return, Pressure and Gravity Feed.
- Gravity Return Filters are pump fed to the filter, but use gravity to return the water to the pond. Therefore, the filter outlet needs to be higher up than the water return point.
- Pressure Filters have the filter unit within a sealed unit, and therefore the water is pressured as it goes to the filter and returned to the pond. This means you have more flexibility on where to position the filter.
- Gravity Feed Filters have the filter sunk into the ground, using gravity to enable water to enter the filter, and a pump to return it. Ideal to power a water jet, waterfall etc., this complex pump system is usually used in large koi ponds.
See Also:
Building a Pond
Introducing Fish to your Pond
Maintaining your Pond
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