Filtration is the heart of every freshwater aquarium. Whether you’re keeping tropical community fish, aquascapes, goldfish, cichlids or shrimp, an effective filtration system is essential for maintaining water quality, supporting beneficial bacteria and creating a healthy environment for aquatic life.

This guide explains how freshwater aquarium filtration works, the different types of filtration available, and how to choose the best filter for your aquarium.

Why Filtration Is Important

In nature, lakes, rivers and streams have enormous volumes of water that naturally dilute waste products. In an aquarium, waste accumulates quickly and must be managed effectively.

A filtration system helps:

✔ Remove debris

✔ Process harmful ammonia

✔ Maintain water clarity

✔ Improve oxygenation

✔ Support beneficial bacteria

✔ Reduce maintenance requirements

✔ Promote fish health

Without adequate filtration, water quality can deteriorate rapidly.

The Three Types of Aquarium Filtration

Most freshwater filters perform three key functions:

Mechanical Filtration

Biological Filtration

Chemical Filtration

Understanding these filtration stages is essential when choosing a filter.

Mechanical Filtration

Mechanical filtration physically removes solid waste from the water.

Examples include:

• Fish waste
• Uneaten food
• Plant debris
• Dust particles

How Mechanical Filtration Works

As water passes through filter media, larger particles become trapped.

Common mechanical media include:

Filter Sponge

Filter Floss

Fine Filter Pads

Coarse Foam Blocks

These materials help keep aquarium water clear and prevent debris from accumulating.

Benefits of Mechanical Filtration

✔ Clearer water

✔ Reduced organic waste

✔ Improved filter performance

✔ Better water quality

Mechanical media should be cleaned regularly to prevent blockages.

Biological Filtration

Biological filtration is the most important form of filtration in a freshwater aquarium.

It is responsible for processing toxic waste products produced by fish and other aquarium inhabitants.

Understanding the Nitrogen Cycle

Fish constantly produce waste.

This waste breaks down into:

Ammonia (NH₃)

Ammonia is highly toxic to fish.

Beneficial bacteria convert ammonia into:

Nitrite (NO₂)

Nitrite is also toxic.

A second group of beneficial bacteria then converts nitrite into:

Nitrate (NO₃)

Nitrate is far less harmful and can be controlled through water changes and plant growth.

Biological Filter Media

Biological media provides a large surface area for beneficial bacteria to colonise.

Popular options include:

Ceramic Rings

Sintered Glass Media

Bio Balls

Porous Stone Media

Biological Sponges

The greater the surface area, the larger the bacterial population the filter can support.

Benefits of Biological Filtration

✔ Removes toxic ammonia

✔ Removes nitrite

✔ Stabilises water quality

✔ Supports healthy fish

✔ Essential for long-term aquarium success

Chemical Filtration

Chemical filtration removes dissolved substances from aquarium water.

Unlike mechanical and biological filtration, it is usually optional.

Activated Carbon

Activated carbon is one of the most common chemical filter media.

It helps remove:

• Discolouration
• Odours
• Medication residues
• Organic compounds

Benefits

✔ Crystal clear water

✔ Improved water quality

✔ Removes contaminants

Specialty Chemical Media

Other chemical media may target specific issues:

Phosphate Removers

Ammonia Absorbers

Nitrate Reduction Media

Tannin Removal Media

These are typically used to solve specific water quality problems.

Types of Freshwater Aquarium Filters

Several filter types are available, each suited to different aquarium sizes and applications.

Internal Filters

Internal filters sit inside the aquarium.

Benefits

✔ Easy installation

✔ Affordable

✔ Suitable for small aquariums

✔ Low maintenance

Ideal For

• Beginner aquariums
• Nano tanks
• Small tropical setups

Hang-On-Back (HOB) Filters

HOB filters hang on the rear of the aquarium.

Water is drawn through filter media before returning to the tank.

Benefits

✔ Compact design

✔ Easy access

✔ Good filtration capacity

✔ Popular in planted aquariums

Ideal For

• Small to medium aquariums
• Tropical community tanks

External Canister Filters

External filters sit outside the aquarium, usually within the cabinet.

Water is pumped through multiple stages of filtration.

Benefits

✔ Large media capacity

✔ Excellent filtration performance

✔ Flexible media configuration

✔ Minimal visual impact

Ideal For

• Medium and large aquariums
• Planted tanks
• Cichlid aquariums
• Heavily stocked systems

Canister filters are often considered the premium filtration solution for freshwater aquariums.

Sponge Filters

Sponge filters use air-driven circulation to draw water through a foam sponge.

Benefits

✔ Gentle flow

✔ Excellent biological filtration

✔ Safe for fry and shrimp

✔ Extremely reliable

Ideal For

• Shrimp tanks
• Breeding aquariums
• Fry rearing systems

Undergravel Filters

Undergravel filters draw water through the substrate.

While less common today, they can still provide effective biological filtration in certain setups.

Benefits

✔ Simple design

✔ Hidden filtration

✔ Large biological surface area

Limitations

• Difficult maintenance
• Less popular than modern filter designs

Filtration for Planted Aquariums

Planted aquariums have unique filtration requirements.

Aquatic plants help consume:

• Ammonia
• Nitrate
• Organic waste

As a result, heavily planted aquariums often require less aggressive filtration.

Recommended Options

• External canister filters
• Hang-on-back filters
• Gentle circulation systems

Excessive flow can sometimes be detrimental to delicate plants.

Filtration for Goldfish Aquariums

Goldfish produce large amounts of waste.

This means they require:

Oversized filtration

Strong biological capacity

Frequent maintenance

External canister filters are often recommended for goldfish systems.

Filtration for Cichlid Aquariums

Cichlids can create heavy bioloads.

Recommended features include:

✔ Strong mechanical filtration

✔ Large biological media volume

✔ High water turnover

External filters are particularly popular for African and Central American cichlid aquariums.

Filtration for Shrimp Aquariums

Shrimp require gentle water movement.

Popular options include:

Sponge Filters

Small Internal Filters

Shrimp-Safe Canister Filters

Intakes should be protected to prevent shrimp from being drawn into the filter.

Understanding Filter Turnover

Turnover refers to how many times the aquarium volume passes through the filter each hour.

For example:

A 200-litre aquarium with a 1,000 LPH filter provides: 5x turnover

General Guidelines

Tropical Community Tanks: 4–6x turnover

Planted Aquariums: 5–8x turnover

Goldfish Tanks: 8–10x turnover

Cichlid Tanks: 8–12x turnover

These figures provide a useful starting point.

Maintaining Your Filter

Regular maintenance helps maintain performance.

Mechanical Media

Clean every: 1–4 weeks

depending on stocking levels.

Use aquarium water rather than tap water whenever possible.

Biological Media

Clean only when necessary.

Avoid excessive cleaning as this can damage beneficial bacteria populations.

Chemical Media

Replace according to manufacturer recommendations.

Activated carbon typically lasts: 4–8 weeks

depending on usage.

Common Filtration Mistakes

Overcleaning Biological Media

This can disrupt beneficial bacteria and destabilise the nitrogen cycle.

Replacing All Media at Once

Always preserve some mature media whenever possible.

Undersized Filtration

Many aquariums benefit from larger filters than the minimum recommendation.

Ignoring Maintenance

Dirty mechanical media can restrict flow and reduce filtration performance.

Rinsing Media Under Tap Water

Chlorine and chloramine can harm beneficial bacteria.

Always use aquarium water where possible.

Signs Your Aquarium Needs Better Filtration

Watch for:

• Cloudy water
• Elevated ammonia
• Elevated nitrite
• Excess waste accumulation
• Persistent algae problems
• Poor fish health

These can indicate insufficient filtration or maintenance.

Choosing the Right Filter

When selecting a filter, consider:

Aquarium Size

Livestock Type

Stocking Density

Maintenance Preferences

Future Growth Plans

For most aquariums, choosing a slightly larger filter provides additional stability and flexibility.

Final Thoughts

Filtration is one of the most important investments you can make in a freshwater aquarium. A quality filter not only keeps water clear but also supports the beneficial bacteria that make aquatic life possible. By understanding mechanical, biological and chemical filtration, you can choose the best filtration system for your aquarium and maintain a healthy environment for fish, plants and invertebrates.

Whether you’re setting up a small shrimp tank, a heavily planted aquascape, or a large cichlid aquarium, selecting the right filtration system and maintaining it properly will significantly improve long-term success.

Need Help Choosing a Freshwater Aquarium Filter?

At Charterhouse Aquatics, we stock a wide range of freshwater filtration solutions including internal filters, external canister filters, hang-on-back filters, sponge filters, filter media, replacement parts and maintenance accessories from leading brands including EHEIM, OASE, Fluval, JBL, Aquael and Sicce. Our aquarium specialists can help you select the ideal filtration system for your freshwater aquarium.