Biological filtration is the most important kind of filtration in your aquarium. What exactly is biological filtration? How does it work? How do you use it?

What is Biological Filtration?
Uneaten fish food, fish excretions, and dead plant, fish, and coral debris accumulate in aquariums. These materials are unattractive and can break down into dissolved toxins such as ammonia and nitrate. Ammonia and nitrate will kill fish and corals.Biological filtration in aquariums is the natural process that converts unwanted materials into non-harmful materials and gases. This process is commonly called the nitrogen cycle.

How Does Biological Filtration Work?
Unwanted material most often enters the aquarium when you feed your fish. Some fish food is uneaten, and some is eaten and excreted by fish. Uneaten food and fish excretions decay into toxic compounds that can poison your aquarium inhabitants.

Biological filtration can start with scavenging animals (e.g., shrimp, snails, crabs) that eat uneaten food and fish waste. Keep in mind that these scavenger animals will excrete waste that dissolves into toxic ammonia and phosphorus, but the net amount of ammonia and phosphorus will be less.

Aerobic bacteria consume ammonia and convert it into nitrite, and then more aerobic bacteria consume the nitrite and turn it into nitrate. Aerobic bacteria are bacteria that live in highly oxygenated areas.

After nitrite is converted to nitrate, anaerobic bacteria consume nitrate along with phosphorus and carbon to convert it into nitrogen gas. Nitrogen gas leaves the aquarium and reenters the atmosphere. Anaerobic bacteria live in low-oxygen areas.

While ammonia, nitrite, and nitrate are harmful to fish and corals, they are needed for freshwater plant growth. Ammonia, nitrite, nitrate, and phosphorus are fertilizers for algae and plants.

How to Use Biological Filtration?
As mentioned in the previous section, using bacteria, scavenger animals, algae, and plants (freshwater) can all be part of a biological filtration plan.

Utilizing Bacteria
Natural Bacteria
Biological filtration using bacteria will occur regardless of what you do. Bacteria exist everywhere. It’s in the air, in the water, and everywhere else. It will live in the sand, rocks, and on the glass of any aquarium.

The trick with an aquarium is to create as much surface area as possible. The more surface area, the more bacteria. The simplest way to do this is to use bio media or a bio grid. This media often comes with canister filters and power filters, and can usually be added to a canister filter or sump. This media is called bio balls, bio media, bio rings, or ceramic rings. For even more surface area, you could use an advanced biomedia such as Brightwell Aquatics Xport Bio Bricks.

Jump Starting Bacteria
Although you will probably eventually have sufficient bacteria with standard bio media, when you start a new tank, the accumulated bacteria may be inadequate. We recommend using Brightwell Aquatics MicroBacter7 with new tanks.

Carbon Dosing
Carbon dosing is a way to turbocharge the anaerobic bacteria that remove nitrate and phosphate. Carbon dosing can be done using NPX Bioplastics in a reactor.

Further reading: Organic Carbon Dosing Article. We also have a Carbon Dosing Video.

Utilizing Algae & Plants
Saltwater
In saltwater aquariums, algae such as calurpa or chaeto help biological filtration because they use toxic waste as fertilizer. Because algae is considered unattractive, it is usually grown in a refugium (a term derived from its original use as a refuge for organisms that can’t survive in the main aquarium). We carry the FijiCube Refugiums.

Further reading: Refugium Overview Article

Freshwater
Ammonia, nitrite, nitrate, and phosphorus are all excellent plant fertilizers. The more plants, the better.

Utilizing Scavengers
It is important to include a cleanup crew in your aquarium. A cleanup crew consists of the scavenging animals that eat uneaten food and fish waste. In freshwater aquariums, adding loaches, shrimp, snails, and corydora catfish will help. In saltwater, adding snails, crabs, shrimp, brittle starfish, and sand-sifting gobies will help.

The Relationship Between Biological Filtration & Mechanical Filtration
The more often you replace your mechanical filter media, the less waste your system will produce that needs to be removed by biological filtration. In reef aquariums, removing waste is best done by protein skimmers.