A bottom filter, also known as an undergravel filter (UGF), is a simple yet effective type of aquarium filter that uses the gravel substrate itself as the primary filter medium.
Understanding the Components
At its core, a bottom filter system is made up of a few key parts:
- Filter Plate: This is a flat, rectangular plastic plate designed to sit directly on the bottom of your tank. It typically has many slots or holes in its surface.
- Uplift Tubes: These vertical tubes attach to the filter plate and extend upwards towards the water surface.
- Power Source: This can be either an air pump connected via airline tubing and an air stone placed inside the uplift tube, or a powerhead water pump attached to the top of the tube.
Here's a quick look at the components:
| Component | Description | Function |
|---|---|---|
| Filter Plate | Slotted plastic plate on tank bottom | Supports gravel, allows water flow |
| Uplift Tubes | Vertical tubes connected to plate | Directs water flow upwards |
| Power Source | Air pump with air stone or powerhead water pump | Creates suction/push to move water |
| Gravel/Substrate | Layer of gravel covering the plate | Acts as the primary filter medium |
The Filtration Process Explained
The core function of a bottom filter relies on creating a flow of water through the gravel substrate.
As the reference states, "The tube pulls water down through the gravel and under the plate, then back up the tube."
Here's a breakdown of this process:
- Creating Suction: The power source (either bubbles rising from an air stone or the impeller of a powerhead) inside the uplift tube draws water upwards and out of the tube. This creates an area of lower pressure (suction) at the base of the tube and under the filter plate.
- Water Movement: The suction pulls water down from the main body of the aquarium. However, because the filter plate covers the bottom, the water is forced to pass through the layer of gravel sitting on top of the plate.
- Filtration: As the water moves down through the gravel, it is filtered in two main ways:
- Mechanical Filtration: Larger debris and particles in the water are trapped by the spaces between the gravel pieces.
- Biological Filtration: This is where undergravel filters truly shine. The vast surface area of the gravel provides an ideal environment for beneficial bacteria to colonize. As water passes through the gravel, these bacteria consume harmful ammonia and nitrite, converting them into less toxic nitrate.
- Return to Tank: After passing through the gravel and under the filter plate, the filtered water is drawn into the base of the uplift tube and is then pushed back up the tube and into the main aquarium environment, completing the cycle.
Practical Considerations
- Gravel Size: A gravel size of about 3-5mm is generally recommended for effective filtration without becoming easily clogged.
- Maintenance: While effective for biological filtration, undergravel filters can accumulate debris under the plate over time, requiring periodic deep cleaning (often involving vacuuming the gravel).
- Combined Filtration: UGFs are often used in conjunction with other filter types, like hang-on-back or canister filters, to provide comprehensive mechanical and chemical filtration as well.
In essence, a bottom filter leverages the natural structure of the aquarium and its substrate to create a simple, efficient filtration system, particularly strong in supporting a healthy biological filter bed.
