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Choosing the right aquarium filter is one of the most important decisions you’ll make when setting up a freshwater aquarium. A quality internal filter not only keeps your water crystal clear but also provides essential biological filtration and water movement, creating a healthy environment where fish and plants can thrive.
Internal filters remain one of the most popular filtration choices thanks to their affordability, ease of installation, and reliable performance. They’re particularly well suited to small and medium-sized aquariums, making them an excellent option for beginners and experienced fishkeepers alike.
In this guide, we’ll explain how internal filters work, their advantages and disadvantages, and how to choose the right model for your aquarium.
What Is an Internal Aquarium Filter?
An internal filter is a self-contained filtration unit that sits inside the aquarium and operates while fully submerged.
Water is drawn into the filter, passes through various filter media, and is returned to the aquarium clean and oxygenated.
Most internal filters combine:
- Mechanical filtration
- Biological filtration
- Water circulation
- Surface agitation
Some models also provide chemical filtration using activated carbon or specialised media.
How Does an Internal Filter Work?
Although designs vary slightly, most internal filters follow the same basic process.
- Water is drawn into the filter through intake slots.
- Mechanical media traps solid waste.
- Biological media houses beneficial bacteria.
- Optional chemical media removes impurities.
- Clean water is returned to the aquarium.
This continuous cycle helps maintain excellent water quality.
The Three Types of Filtration
A good internal filter should provide all three forms of filtration.
Mechanical Filtration
Mechanical filtration removes physical debris such as:
- Fish waste
- Uneaten food
- Plant debris
- Floating particles
Filter sponges are the most common mechanical media.
Keeping mechanical media clean helps maintain strong water flow.
Biological Filtration
Biological filtration is the most important function of any aquarium filter.
Beneficial bacteria colonise porous filter media and convert:
- Ammonia → Nitrite → Nitrate
This process, known as the nitrogen cycle, protects fish from harmful toxins.
Never replace all biological media at once, as this can remove the bacteria your aquarium relies on.
Chemical Filtration
Some internal filters include space for chemical media such as:
- Activated carbon
- Phosphate remover
- Ammonia-removing resin
Chemical media can improve water clarity and remove dissolved impurities but is generally optional for most freshwater aquariums.
Why Choose an Internal Filter?
Internal filters offer several advantages.
Easy Installation
Most models simply attach to the aquarium glass using suction cups.
No plumbing or pipework is required.
Affordable
Internal filters are generally less expensive than external canister filters, making them ideal for first-time fishkeepers.
Compact Design
Modern internal filters take up relatively little space and fit neatly into the corner of most aquariums.
Quiet Operation
Quality internal filters are usually very quiet when properly maintained.
Reliable Performance
With fewer external hoses and seals, there is little risk of leaks.
Are Internal Filters Suitable for Every Aquarium?
Internal filters are ideal for:
- Tropical freshwater aquariums
- Coldwater aquariums
- Shrimp tanks
- Planted aquariums
- Quarantine tanks
- Hospital aquariums
However, they may be less suitable for:
- Large aquariums
- Heavily stocked aquariums
- Large cichlid tanks
- Marine reef aquariums
These setups often benefit from larger external filtration systems.
Choosing the Correct Filter Size
One of the most common mistakes is buying a filter that’s too small.
Manufacturers usually recommend an aquarium size for each model.
Always choose a filter suitable for your aquarium’s total water volume.
If you’re between sizes, selecting the larger model is often beneficial.
Understanding Flow Rate
Flow rate refers to how much water the filter moves each hour.
It is usually measured in:
Litres per hour (LPH)
As a general guide:
Tropical Community Aquariums
Aim for approximately:
4–6 times the aquarium volume per hour.
Example:
A 100-litre aquarium should have a filter rated between:
400–600 LPH
Goldfish Aquariums
Goldfish produce significantly more waste.
Aim for:
6–10 times the aquarium volume per hour.
Planted Aquariums
Moderate flow is generally preferred to avoid disturbing delicate plants.
Aquarium Shape Matters
Tall aquariums may require a filter with greater lift to circulate water effectively.
Long aquariums benefit from good flow across the entire tank to prevent dead spots.
Position the outlet to maximise circulation throughout the aquarium.
Types of Internal Filters
Sponge Internal Filters
Air-powered sponge filters are ideal for:
- Fry tanks
- Shrimp aquariums
- Quarantine tanks
Advantages include:
- Gentle water flow
- Excellent biological filtration
- Safe for baby fish
Compact Power Filters
These combine an electric pump with filter media.
Suitable for:
- Small tropical aquariums
- Community tanks
- Beginner setups
Multi-Stage Internal Filters
Premium models contain several media compartments for:
- Mechanical filtration
- Biological filtration
- Chemical filtration
These provide excellent overall filtration and are suitable for larger aquariums.
Adjustable Flow
Many modern filters feature adjustable flow control.
This is particularly useful for:
- Bettas
- Shrimp
- Small tetras
- Long-finned fish
Reducing excessive current helps minimise stress.
Spray Bars and Directional Outlets
Some internal filters include adjustable outlets or spray bars.
These allow you to direct water movement where it’s needed most.
Benefits include:
- Better circulation
- Improved oxygenation
- Reduced dead spots
Internal Filters and Oxygen
Surface movement created by the filter helps increase oxygen exchange.
Good oxygenation benefits:
- Fish
- Beneficial bacteria
- Plants (during darkness)
Avoid pointing the outlet completely underwater with no surface agitation.
Filter Media
Quality filter media makes a significant difference.
Common media include:
Filter Sponge
Excellent mechanical and biological filtration.
Ceramic Rings
Highly porous media supporting large bacterial colonies.
Bio Balls
Provide additional biological filtration.
Activated Carbon
Improves water clarity and removes dissolved organics.
Fine Filter Wool
Produces exceptionally clear water by trapping fine particles.
Cleaning an Internal Filter
Regular maintenance keeps your filter operating efficiently.
Generally:
- Clean mechanical media every 2–4 weeks.
- Rinse media using old aquarium water.
- Clean the impeller every few months.
Avoid replacing filter media unless it is physically deteriorating.
Cleaning the Impeller
Over time, algae, calcium, and debris build up around the impeller.
Cleaning it regularly helps maintain:
- Water flow
- Quiet operation
- Energy efficiency
Use a soft brush and warm water to remove deposits.
Internal Filter Placement
For best performance:
- Position the filter near one end of the aquarium.
- Direct the outlet across the tank.
- Ensure good surface movement.
Avoid hiding the filter completely behind decorations where circulation may be restricted.
Internal Filters and Planted Aquariums
Internal filters work well in planted tanks.
Choose moderate flow to avoid:
- Uprooting plants
- Excessive CO₂ loss
- Strong currents
Many aquascapers use compact internal filters in smaller layouts.
Common Mistakes
Buying Too Small
Undersized filters struggle to maintain water quality.
Always follow the manufacturer’s aquarium size recommendations.
Replacing All Media
Replacing every sponge at once removes beneficial bacteria.
Clean media instead of replacing it wherever possible.
Cleaning with Tap Water
Tap water containing chlorine may damage beneficial bacteria.
Always rinse filter media using water removed during a water change.
Ignoring Maintenance
Reduced flow often indicates:
- Dirty sponges
- Blocked impeller
- Clogged intake
Routine cleaning prevents most problems.
Turning the Filter Off
Beneficial bacteria require oxygenated water.
Avoid leaving the filter switched off for extended periods.
Popular Internal Filter Brands
Some of the most trusted manufacturers include:
- Fluval
- Eheim
- Oase
- Aquael
- Sicce
- SuperFish
- Juwel
- Aqua One
- Marina
Choosing a quality brand often means quieter operation, better reliability, and readily available spare parts.
When Should You Upgrade?
You may wish to consider a larger filter or an external canister filter if:
- Your aquarium becomes heavily stocked.
- Water quality becomes difficult to maintain.
- You require additional filter media capacity.
- You upgrade to a much larger aquarium.
Internal filters are excellent for many setups, but larger aquariums often benefit from increased filtration volume.
Final Thoughts
Internal filters remain one of the most practical and reliable filtration options for freshwater aquariums. They’re affordable, easy to install, simple to maintain, and provide excellent mechanical and biological filtration for a wide range of aquarium sizes.
When choosing an internal filter, focus on selecting a model that’s appropriately sized for your aquarium, offers sufficient media capacity, and provides a suitable flow rate for your fish and plants. Combined with regular maintenance and proper cleaning, a quality internal filter will help maintain crystal-clear water and a healthy environment for years to come.
At Charterhouse Aquatics, we stock a wide range of internal aquarium filters and replacement media from trusted brands including Fluval, Eheim, Oase, Aquael, Sicce, Juwel, SuperFish, and Aqua One. Whether you’re setting up your first tropical aquarium or upgrading an existing system, our experienced team is here to help you choose the perfect filtration solution.
