How To Build A Biofilter OR Life and Trophic Cascades

Ever wonder why maintaining a fish tank takes so much work while natural ponds, lakes, and streams require zero human intervention? One word – ecology. So let’s take a page from nature and establish a biofilter to maintain a clean, clear, biodiverse pond. But let’s first look at two ecological principals.

Principle 1: If there’s a niche, nature will fill it
Principle 2: Life creates conditions conducive to life

To understand how these two principals apply, let’s look at the natural cycling process of a typical pond or aquarium.

The Nitrogen Cycle

Stage 1: Fish

You decide to keep fish. Maybe you’re hungry, maybe you’re looking for companionships. In either case, you build a pond/aquarium, stock it with fish, and pick up some chow. You’re careful not to over feed them but every time you do your fishy companions convert most of it into fish poop.

Stage 2: Ammonia (NH3) Eating Bacteria 

Unfortunately for the fish, fish waste is high in ammonia – a toxic compound created by the metabolization of protein.  Ammonia displaces oxygen, causes organs to shut down, and makes fish susceptible to a host of diseases. Ammonia’s toxic effect is why it’s so effective as a household cleaner. Rub a little ammonia on a countertop and it’ll kill anything living there. It’s not the kind of stuff you’d like to swim in.

If ammonia’s so terrible, why doesn’t it build up and kill everything? The answer is nitrifying bacteria. Some bacteria, as it turns consume ammonia as food (Principle 1: if there’s a niche, nature will fill it). These ammonia eating bacteria are naturally occurring and will find your fish tank on their own, though, you can speed up the process by adding nitrifying bacteria purchased at any fish store. A good product that I have used many times is Stability. You can also add stones or filter medium from a previously established fish tank.

Stage 3: Nitrite (NO2-) Eating Bacteria

Once ammonia consuming bacteria establish themselves, ammonia levels will drop to next to zero. At this point, bacteria are consuming ammonia as fast as the fish can produce it. In a very real way, fish produce food for the bacteria (Principle 2: life creates conditions conducive to life). Unfortunately, our helpful bacteria convert ammonia into a second toxic compound – nitrite. Though just as before, a waste consuming species discovers this food rich resource and sets up shop (Principle 1: if there’s a niche, nature will fill it).

Stage 4: Nitrate (NO3-) Eating Plants

After being consumed a second time, our toxic waste is finally converted to nitrate – a more or less benign compound. With ammonia and nitrite levels at zero, there’s no immediate threat to fish, though, if you’ve kept fish, you’ve probably spent hours scraping slimy algae from rocks and walls – that’s because nitrate makes an excellent plant fertilizer.

Like the nitrifying bacteria before it, algae set up shop to take advantage of this unused resource (Principle 1: if there’s a niche, nature will fill it). In a typical pond or aquarium, this is usually where we stop…left cleaning, changing water, and/or adding chemicals to keep the pond clear. It doesn’t have to be this way.

Stage 5: Plant Eating Animals

Algae are microscopic plants and almost guarenteed to search out and find your nitrate rish fish tank – so why not include plants them intentionally? Ornamental aquatic plants are an obvious choice but there are other options. If your fish are herbatious, grow them food from their own waste – duckweed is a good option. If you eat plants, consider dabbeling in aquaponics – here’s a DIY aquaponics system that sits on top of your fish tank. The bottom line is, if you can encouage plants to live if you’re system, they will naturally remove the nitate so you wont have to. – that means no 1/3 water changes or scummy algea buildup.

Stages 6+

At each stage in the process, a new organism was added to take advantage of the waste produces in the previous stage. In a very real sense, the system moves through a type of succession. Not only did each stage bring new life (Principle 1: if there’s a niche, nature will fill it) but it also brought opportunities for new types of organisms (Principle 2: life creates conditions conducive to life). In our aquapic system the fish are a keystone species that enables a whole host of other organisms to flourish. In ecological terms, we call this a trophic cascade. If you remove the fish, the entire system would collapse.

A Tangent On Trophic Cascades

A departure from fish but here’s a fantastic video from George Monbiot (4:34) on how wolves (and the trophic cascades they create) changed the ecology of Yellowstone National Park.

Building a Biofilter for Your Pond or Fish Tank

In order to create an effective biofilter you will need a lot of two things – surface area and water movement… and that’s pretty much it.

The nitrifying bacteria mentioned in stages 2 and 3 tend to form dense colonies growing on any surface that they can cling to. In the wild this means rocks, sticks, plants etc, though, it could also be the surface of your fish tank, the pebbles at the bottom, plumbing, etc. In either case, the more surface area your pond has the more bacteria can colonize it. You can go to aquarium stores or online and  buy materials such as bioballs or expanded clay pellets but any non-toxic surface will do. I like kitchen sponges.

Water movement is important to ensure that your bacteria is coming in contact with the ammonia and nitrate it’ll be eating. Adding a pump to circulate water through a biofilter will ensure adequate contact. As a rule of thumb, I like to circulate my entire pond about once an hour – determine how much water your system holds then pair it with a submersible pump capable or pushing that volume of water per hour.

Biofilter From An IBS Tote

In the system below, I filled a 275-gallon IBS tote with sponges and bio-ribbon (purchased from an aquarium store) to create my surface area.

I then ran two 1200 gallon per hour pumps from the pond into the bottom of the IBS tote biofilter. As the water level rises inside the tote, it overflows though a 2 inch pipe I plumbed into the top of the tote. Adding water the the bottom of the biofilter and overflowing from the top forces the water to move through the surface area medium inside the biofilter.

Pond biofilter made from tote.

DIY biofilter for an indoor koi pond.

Biofilter outlet back to pond.

Ammonia and Nitrite consuming bacteria will colonize any surface area.