Filtration

The Vital Role of Filtration in Fishkeeping: A Guide to Filter Types

Filtration is one of the most critical components of a successful and healthy aquarium. Whether you’re keeping a simple goldfish tank or a complex tropical aquascape, proper filtration ensures clean water, removes harmful substances, and provides a stable environment for your fish to thrive.

Filtration does far more than just clear up murky water—it drives the nitrogen cycle, supports beneficial bacteria, and helps maintain oxygen levels. Without effective filtration, toxic compounds such as ammonia and nitrite build up quickly, leading to stress, illness, or even death in fish.

In this article, we’ll explore the three main types of filtration—mechanical, biological, and chemical—and compare four of the most commonly used filter systems in home aquariums:

• External Canister Filters
• Sponge Filters
• Undergravel Filters
• Hang-on-Back (HOB) Filters

Understanding the Types of Filtration

• Mechanical Filtration: Physically removes debris and suspended particles from the water using filter media like sponges or floss.
• Biological Filtration: Provides a home for beneficial bacteria that convert toxic ammonia (from fish waste) into nitrite and then into less harmful nitrate via the nitrogen cycle.
• Chemical Filtration: Uses substances like activated carbon or resin to remove dissolved organic compounds, odours, and discolouration.

1. External Canister Filters

Canister filters are highly efficient, external units that sit outside the tank and push water through various media layers using a powerful pump. These are commonly used in larger aquariums due to their excellent performance and customisability.

Pros:

• Extremely powerful—ideal for medium to large tanks (100L+)
• Supports all three types of filtration (mechanical, biological, chemical)
• Quiet operation as it runs externally
• Media can be customised for specific needs (e.g. carbon, ceramic rings, fine floss)
• Doesn’t take up space inside the aquarium

Cons:

• Generally more expensive than internal filters
• Requires more effort to clean (disconnect hoses, open unit)
• Initial setup can be time-consuming

2. Sponge Filters

Sponge filters are air-driven filters that provide gentle filtration using a sponge as the main media. They are commonly used in breeding tanks, fry tanks, and shrimp tanks where water flow needs to be gentle.

Pros:

• Excellent biological filtration through the sponge surface
• Safe for small or delicate livestock (e.g. fry or shrimp)
• Very affordable and easy to maintain
• Minimal moving parts—long-lasting

Cons:

• Limited mechanical filtration; doesn’t polish water well
• Does not include chemical filtration
• Driven by an air pump, which can be noisy
• Less suitable for larger or heavily stocked tanks

3. Undergravel Filters

Undergravel filters use a plate under the substrate and draw water through the gravel bed using uplift tubes and an air pump or powerhead. They rely on the gravel itself acting as a biological filter.

Pros:

• Low initial cost and discreet appearance
• Uses the tank substrate for biological filtration
• Simple technology with minimal equipment

Cons:

• Hard to clean and maintain; detritus builds under the plate
• Can clog over time, reducing effectiveness
• Limited mechanical and no chemical filtration
• Not ideal for planted tanks or fine substrates

4. Hang-on-Back (HOB) Filters

HOB filters hang on the back of the aquarium and use a siphon tube to draw water up and filter it through cartridges or media trays before returning it via a waterfall-style output.

Pros:

• Easy to install and maintain—media cartridges are quick to replace
• Provides mechanical, biological, and often chemical filtration
• Affordable and widely available
• Great for tanks up to 200L

Cons:

• Can be noisy due to waterfall return and motor hum
• Takes up some space on the back of the tank
• Cartridges may need frequent replacement (cost over time)
• Less customisable than canisters

Choosing the Right Filter for Your Aquarium

When selecting a filter, consider the size of your aquarium, the type of fish you keep, your budget, and your maintenance preferences. Here’s a quick comparison:

Final Thoughts

Filtration is the backbone of a healthy aquarium. It not only keeps your tank clear and pleasant to look at, but it also performs life-saving functions for your fish. Choosing the right filter—and maintaining it properly—ensures stable water conditions, less disease, and more enjoyment from your aquatic hobby.

Whether you go for a canister, sponge, undergravel, or hang-on-back filter, make sure it suits your aquarium’s size, livestock, and your personal maintenance habits. Your fish will thank you with long lives and vibrant colours.