Pond Filtration 101: A Beginner's Guide

Pond Filtration 101: A Comprehensive Beginner's Guide

Backyard ponds are enchanting havens of tranquility, offering a slice of nature's beauty right in your own outdoor space. However, maintaining a healthy and vibrant pond ecosystem requires proper filtration. In this beginner's guide, we'll delve into the realm of backyard pond filtration, exploring its important role, and explaining how to keep your filter working efficiently.

Understanding Pond Filtration

Many pond owners wonder why their water garden or fishpond needs a filter since natural ponds don’t have filters. The main reason backyard ponds need a filter is that natural ponds have much less fish and plants per gallon of water than natural ponds. Natural ponds are much deeper and can recycle fish waste and plant materials more efficiently than man-made water gardens and goldfish ponds. 

A pond filter keeps your pond clean and clear and improves water quality for your fish and plants. It is important to understand the fundamentals of pond filtration. It will help you select the proper pond filter and keep it operating at peak performance.

It All Begins with the Water Pump

All pond filters operate on the same basic principle: a water pump circulates pond water through the filter system and back into the pond. Some filter designs use the pump to push water into a filter. Others pull water through the filter and out of the water pump. 

  • Submerged filters sit on the bottom of the pond. The pump pulls water through the filter and pushes it up to a fountain and waterfall.
  • External filters use a submerged pump to move pond water out of the pond and into a filter that sits outside the pond. Filtered water flows back into the pond, often in the form of a waterfall.
  • Advanced external filters, used on large ponds, use a “skimmer” that draws water from the surface of the pond. A pond skimmer is like a swimming pool skimmer. It keeps the surface of the water free of pollen, leaves and other floating debris.

No matter what type of filter you have, the water pump makes it work by bringing water to the filter media.

Types of Pond Filtration Media

Pond filtration isn't a one-size-fits-all solution. There are several filtration methods and equipment options, each with specific properties. In this section, we'll break down the various types of pond filtration systems, shedding light on their distinct functions and advantages. Not all pond filters are the same!

Mechanical Marvels: The Power of Mechanical Filtration

Mechanical filtration plays a crucial role as the primary line of defense in maintaining a clear pond. The presence of suspended particles such as soil fragments, plant debris, and other floating matter often leads to cloudy water, making it challenging to observe your fish. Green water, caused by algae, will be addressed later in this article. To combat the issue of dirty, particle-filled water and enhance visibility, mechanical filtration steps in.

Much like the filters in your home's heating and air conditioning system, mechanical filtration removes solid particles from the pond water. Various forms of mechanical filtration include:

  • Porous sponge blocks
  • Pre-cut sponge pads
  • Pre-cut polyester pads
  • DIY rolls of polyester filter material

The key takeaway is that mechanical filtration effectively traps debris. However, as the filter material captures debris, it tends to become clogged, resulting in a decrease in the water flow rate through the filter. We will delve into how to maintain optimal water flow in the filter in the maintenance section later on.

Balancing Act: The Science Behind Biological Filtration

You've probably come across the idea that a "balanced" pond is essential and that murky water might indicate a biological imbalance. But what does this actually mean? Let's clarify the term "biological filtration" and distinguish it from a "biological filter."

Biological filtration is a broad term encompassing nature's remarkable recycling system. In your pond, there are billions of invisible microbes tirelessly at work, day and night. Their mission? To break down organic matter like dead plants, algae, leaves that find their way into the water, deceased insects, fish waste, and other natural materials. This natural process occurs both in the wild and in your backyard pond. These microscopic helpers are the cornerstone of a thriving pond ecosystem.

Take fish waste, for instance; it contains ammonia. Typically, this ammonia serves as a "food" source for plants and certain microbes. However, if you overcrowd your pond with too many fish or overfeed them, the ammonia levels can rise to toxic levels for the fish, leading to a biological imbalance. You might also witness excessive algae growth because the surplus fish food disrupts the balance of nutrients in the water. In essence, biological filtration is the continuous action of these beneficial microbes, ensuring that the water quality remains in optimal condition.

What is a Biological Filter?

You’ll often see pond filters advertising biological filtration as a benefit. The pond filter will contain plastic spheres, brushes, and other objects. The idea is that beneficial microbes will attach to the “bio media” and clean the water as it passes through the filter. 

However, here's the truth: the majority of these beneficial microbes naturally reside in the water, on the pond's surfaces, rocks, and even on aquatic plants. Unless you have a specialized fish-only pond densely populated with koi or goldfish, adding biological filtration media to your filter isn't necessary.

One notable advantage, though,  is that the biofilter media create many paths for the water to follow through the filter. This allows floating debris to settle more easily inside the filter, helping to keep the water clear. For most backyard ponds and water gardens, the biological filtration is handled inside the pond, not the filter. Your filter may come with the following biofilter media.

  • Plastic balls
  • Ceramic rings
  • Lava rock
  • Sponge shapes

Biological filter media won’t hurt your pond. In fact, they often end up acting as mechanical filter media, collecting debris as water flows through the media.

Unlocking Clarity: Chemical Filtration's Role

Chemical filtration media works by removing dissolved chemicals from the water. Zeolite, for example, is a natural mineral that removes ammonia. Zeolite is a great emergency treatment if your pond has a high level of ammonia due to overfeeding. Keep in mind that ammonia is colorless in pond water. The only way to know if it is building up is with an ammonia test kit.

Another chemical that can cause problems is phosphate. Phosphate comes from fish waste, uneaten fish food that decays in the pond, tap water, and decaying plants and algae. Phosphate is colorless and can only be detected with a phosphate test kit. Excess phosphate can stimulate algae growth. Granular phosphate removers, packed in a flow-through pouch, are placed inside the pond filter. Phosphate removers can reduce the phosphate level and reduce algae growth. 

Over time your pond water will develop a yellowish tint. This is the result of plants and algae releasing organic substances into the water. While not harmful, it indicates that organic matter is building up in the pond. Many pond keepers make partial water changes in the spring and fall to flush out these tea-colored compounds. Activated carbon is a granular media that can also help to remove these organics. Activated carbon is used just like zeolite and phosphate removers.

Most pond filters do not include chemical filter media. Chemical filtration media is an option that you can use to keep your water quality and clarity in good shape. 

Zapping Green Water: UV Sterilization's Impact

Ultraviolet (UV) sterilization is used to disinfect drinking water and air. When used as part of the pond filtration system, a UV unit will eliminate green water caused by algae floating in the pond water. Despite the term “sterilizer”, UV units don’t sterilize the water. 

Here’s how they work. A UV light is housed inside a water-tight tube or built into an external pond filter. As algae and water flow past the light bulb, the ultraviolet radiation kills the algae. A UV sterilizer won’t kill string algae that is attached to the waterfall, pond liner, or plants. But if you have green water, a UV sterilizer will dramatically improve water clarity.

mastering the pond filtration for clean and healthy pond

Breathing Easy: The Importance of Aeration

It’s no secret fish need oxygen to live a healthy life. Oxygen enters pond water through the surface of the water. Agitation from fountains and waterfalls helps to bring oxygen into the water and release excess carbon dioxide. Moreover, the water pump serves a dual purpose: not only does it circulate water through the filter, but it also stirs the water, promoting aeration in the pond.

This aeration process serves a vital role by supplying oxygen to a diverse array of microorganisms that play a pivotal role in preserving the pond's delicate ecosystem balance.

Mastering the Art of Pond Filtration

Now that we've covered the basics of pond filtration, it's time to master the art. Here are practical tips and strategies for effective backyard pond filtration.

Sizing Matters: Selecting the Right Equipment

When selecting a pond filter be sure to match the filter and pump to your pond’s size. Most pond filters provide a recommended size range along with the proper pump size (flow rate). If the filter is too small, it won’t be effective. Decide if you want a submersible filter or an external filter. If you have a small fishpond or water garden, an internal filter will do the job.

Maintenance Mastery: Keeping It Clean

Regular maintenance is the key to keeping the filter working properly and pond water clean and clear.  The key things to remember are:

  • Mechanical filter media should be cleaned or replaced once a month. Even though water is flowing, the trapped debris will consume oxygen and release algae-promoting nutrients as it decomposes.
  • Biofilter media will also trap debris and sludge. Rinse it with pond or tap water at least once a month.
  • Chemical filter media will trap debris. Rinse them once a month. In most cases, chemical media lasts 1 to 2 months before needing replacement.


We all want our backyard pond to be healthy and beautiful.  A pond filter filtration will keep your pond a serene and thriving haven of beauty. Understanding the basics of filtration and applying effective practices will ensure your pond flourishes for years to come. 

Back to blog

Leave a comment

Please note, comments need to be approved before they are published.

1 of 3