Filtration is one of the most important aspects of maintaining a healthy aquarium. Whether you’re keeping tropical fish, goldfish, shrimp, or a heavily planted aquascape, choosing the right filter can make a huge difference to water quality, livestock health, and the overall success of your aquarium.
Among the various filtration options available, external filters (often called canister filters) are widely regarded as one of the most effective and versatile solutions for freshwater aquariums. They offer excellent filtration capacity, increased flexibility, and cleaner aesthetics compared to many internal filters.
But with so many models, flow rates, and features available, how do you choose the right external filter for your aquarium?
In this guide, we’ll explain everything you need to know to select the ideal external filter for your setup.
What Is an External Filter?
An external filter is a filtration unit that sits outside the aquarium, usually inside the cabinet beneath the tank.
Water is drawn from the aquarium through an intake pipe, passes through multiple layers of filter media inside the unit, and is then returned to the aquarium via a spray bar or outlet nozzle.
Because the filter sits outside the tank, it offers several advantages:
- Larger filtration capacity
- More media options
- Improved aesthetics
- Easier maintenance
- Better water clarity
External filters are particularly popular for medium to large freshwater aquariums.
Why Choose an External Filter?
External filters provide a combination of mechanical, biological, and chemical filtration within a single unit.
Benefits include:
- Crystal-clear water
- Excellent biological filtration
- Reduced visible equipment inside the tank
- Increased water volume
- Greater customisation options
- Quiet operation
For many aquarists, they represent the best all-round filtration solution.
Understanding the Three Types of Filtration
Before choosing a filter, it’s important to understand what filtration actually does.
Mechanical Filtration
Mechanical filtration removes physical debris from the water.
Examples include:
- Fish waste
- Uneaten food
- Plant matter
- Suspended particles
Filter sponges and floss are commonly used for this purpose.
Biological Filtration
Biological filtration is arguably the most important function.
Beneficial bacteria colonise filter media and convert harmful waste products through the nitrogen cycle:
Ammonia → Nitrite → Nitrate
The larger the biological media capacity, the greater the filter’s ability to support a healthy aquarium.
Chemical Filtration
Chemical filtration removes dissolved substances using specialised media such as:
- Activated carbon
- Phosphate removers
- Purigen
- Water polishing resins
Not every aquarium requires chemical filtration, but external filters allow it to be added when needed.
Choosing the Correct Filter Size
One of the most common mistakes is purchasing a filter that is too small.
Manufacturers usually recommend a maximum aquarium size, but these ratings should be treated as guidelines rather than strict rules.
For the best performance, many experienced aquarists choose a filter rated for a slightly larger aquarium than they currently own.
Example
If you have a 200-litre aquarium, consider a filter rated for:
- 250 litres
- 300 litres
- Even larger for heavily stocked tanks
This provides additional filtration capacity and improved long-term stability.
Understanding Flow Rate
Flow rate refers to how much water the filter can move per hour.
It is typically measured in:
- Litres per hour (LPH)
- Gallons per hour (GPH)
As a general guideline:
Community Aquariums
Aim for turnover of approximately:
4–6 times aquarium volume per hour
Planted Aquariums
Aim for:
5–10 times aquarium volume per hour
Goldfish Aquariums
Aim for:
8–10 times aquarium volume per hour
High Bioload Setups
Aim for:
10 times aquarium volume or more
Remember that real-world flow rates are usually lower once media and pipework are installed.
Aquarium Type Matters
The ideal filter depends on what you’re keeping.
Tropical Community Aquariums
Most community tanks benefit from:
- Moderate flow
- Strong biological filtration
- Good mechanical filtration
Planted Aquariums
Planted tanks often require:
- Consistent circulation
- Excellent water clarity
- CO₂-friendly flow patterns
Many aquascapers prefer external filters due to their clean appearance and powerful filtration.
Goldfish Aquariums
Goldfish produce significant waste.
Choose:
- Larger filter capacity
- Higher flow rates
- Plenty of biological media
Shrimp Aquariums
Shrimp tanks require gentle flow.
Some external filters allow adjustable output, making them suitable for sensitive livestock.
Media Capacity Is Important
Not all filters are created equal.
Two filters may have similar flow rates but vastly different media capacities.
Larger media baskets allow:
- More biological media
- Greater filtration efficiency
- Improved water stability
When comparing filters, media volume is often just as important as flow rate.
Noise Levels
Modern external filters are generally very quiet.
However, some models are designed specifically for near-silent operation.
If your aquarium is located in:
- A living room
- Bedroom
- Office
noise levels may be an important consideration.
Premium filters often feature improved pump design and vibration reduction.
Ease of Maintenance
All filters require regular maintenance.
Features that make servicing easier include:
- Quick-release hose connections
- Integrated priming systems
- Easy-access media baskets
- Self-priming pumps
The easier a filter is to maintain, the more likely you’ll keep it operating efficiently.
Energy Efficiency
Because filters run 24 hours a day, energy consumption matters.
Modern external filters often feature:
- Efficient motors
- Lower running costs
- Reduced heat transfer
Over time, energy-efficient filters can save money while maintaining excellent performance.
Built-In Features to Consider
Many modern external filters include useful additional features.
UV Sterilisers
Some models include integrated UV units that help:
- Reduce green water
- Improve water clarity
- Control free-floating algae
Heater Integration
Certain filters can be paired with external heaters, reducing visible equipment inside the aquarium.
Adjustable Flow Control
Allows you to customise circulation to suit your livestock.
Aquarium Cabinet Space
Before purchasing, ensure your cabinet has enough room for:
- The filter body
- Pipework
- Maintenance access
Larger filters provide more capacity but require additional space.
Always check dimensions carefully.
Popular External Filter Brands
Several manufacturers have built excellent reputations within the aquarium hobby.
Popular options include:
OASE
Known for:
- High-quality construction
- Integrated heater options
- Quiet operation
Fluval
Popular for:
- Reliability
- Strong performance
- Wide range of sizes
Eheim
Renowned for:
- Longevity
- Build quality
- Efficient filtration
JBL
Offers:
- Excellent value
- Large media capacity
- Easy maintenance
Aqua One
Provides:
- Budget-friendly solutions
- Reliable filtration
- Beginner-friendly operation
Common Mistakes When Choosing a Filter
Avoid these common pitfalls:
Buying the Smallest Option
Larger filters generally perform better and require less frequent maintenance.
Focusing Only on Flow Rate
Media capacity is equally important.
Ignoring Future Growth
Many aquariums become more heavily stocked over time.
Forgetting Maintenance Access
Ensure you can comfortably remove and service the filter.
Should You Run More Than One Filter?
In some larger aquariums, multiple filters can provide benefits.
Advantages include:
- Increased filtration capacity
- Improved circulation
- Redundancy if one filter fails
- Easier maintenance rotation
Large aquariums often benefit from dual-filter systems.
External Filter Maintenance Tips
To keep your filter operating efficiently:
- Clean mechanical media regularly
- Avoid washing biological media in tap water
- Inspect hoses for blockages
- Check impellers periodically
- Replace worn seals when necessary
Regular maintenance helps ensure reliable performance and extends equipment lifespan.
Final Thoughts
Choosing the right external filter is one of the most important decisions you’ll make when setting up an aquarium. A properly sized filter provides excellent water clarity, stable biological filtration, and healthier conditions for your fish and plants.
When selecting a filter, consider your aquarium size, stocking levels, media capacity requirements, flow rate, maintenance preferences, and available cabinet space. Investing in a quality external filter can make aquarium maintenance easier while providing years of reliable service.
At Charterhouse Aquatics, we stock a wide range of external filters from leading brands including OASE, Fluval, Eheim, JBL, and Aqua One, helping aquarists find the perfect filtration solution for every aquarium.
