A good aquarium filter is the heart of every successful fish tank. While decorations, lighting and plants create an attractive display, it’s the filter that keeps your aquarium healthy by removing waste, circulating water and providing a home for the beneficial bacteria that keep toxic ammonia and nitrite under control.
With so many different types of aquarium filters available—from internal filters and external canister filters to sponge filters and hang-on-back models—it can be difficult to know which one is right for your aquarium.
In this comprehensive guide, we’ll explain how aquarium filters work, compare the different types available, and help you choose the best filtration system for your freshwater or marine aquarium.
Why Do You Need an Aquarium Filter?
An aquarium is a closed ecosystem.
Unlike rivers and lakes, waste doesn’t simply wash away. Fish produce waste continuously, uneaten food decomposes, and plant material breaks down over time.
Without proper filtration, this leads to:
- Ammonia spikes
- Poor water quality
- Algae problems
- Low oxygen levels
- Fish stress
- Disease outbreaks
A quality filter helps keep your aquarium clean, stable and healthy.
What Does an Aquarium Filter Actually Do?
Most aquarium filters perform three different types of filtration.
Mechanical Filtration
Mechanical filtration removes visible particles from the water.
This includes:
- Fish waste
- Uneaten food
- Plant debris
- Floating particles
Sponges, filter floss and fine filter pads trap these particles before they break down.
This helps maintain crystal-clear water.
Biological Filtration
Biological filtration is arguably the most important job a filter performs.
Beneficial bacteria colonise biological media inside the filter and convert harmful waste products through the nitrogen cycle.
They convert:
- Ammonia → Nitrite
- Nitrite → Nitrate
Without biological filtration, fish would quickly be poisoned by their own waste.
Common biological media include:
- Ceramic rings
- Sintered glass
- Bio balls
- Porous rock media
- High-performance biological blocks
The more biological media your filter holds, the greater its filtration capacity.
Chemical Filtration
Chemical filtration removes dissolved substances from the water.
Common media include:
- Activated carbon
- Phosphate remover
- Purigen
- Zeolite
- Heavy metal removers
Chemical filtration is optional in many aquariums but can be extremely useful when dealing with specific water quality issues.
Types of Aquarium Filters
There are several different types of aquarium filters available.
Each has advantages depending on your aquarium and livestock.
Internal Filters
Internal filters sit inside the aquarium itself.
Water is drawn through filter media before being returned to the tank.
Advantages include:
- Affordable
- Easy to install
- Compact
- Ideal for beginners
- Suitable for smaller aquariums
Disadvantages include:
- Takes up space inside the aquarium
- Smaller media capacity
- Limited upgrade options
Best suited for:
- Nano aquariums
- Small tropical tanks
- Goldfish tanks
- Beginner aquariums
External Canister Filters
External filters sit beneath the aquarium inside the cabinet.
Water travels through hoses into the filter before returning to the aquarium.
Advantages include:
- Excellent filtration capacity
- Large biological media volume
- Keeps equipment outside the aquarium
- Easy maintenance
- Crystal-clear water
Disadvantages include:
- Higher purchase cost
- Requires cabinet space
- More complex installation
Best suited for:
- Medium aquariums
- Large aquariums
- Planted tanks
- Goldfish aquariums
- Marine fish-only systems
Many experienced fishkeepers consider external filters the gold standard for freshwater aquariums.
Hang-On-Back (HOB) Filters
Hang-on-back filters attach to the rear of the aquarium.
They draw water into a small filter chamber before returning it via a waterfall outlet.
Advantages:
- Easy installation
- Excellent surface agitation
- Compact design
- Easy access for maintenance
Disadvantages:
- Visible from behind
- Smaller media capacity than external filters
- May increase evaporation
Ideal for:
- Small to medium aquariums
- Shrimp tanks
- Nano aquariums
Sponge Filters
Sponge filters are powered by an air pump.
Water is drawn through the sponge, providing both mechanical and biological filtration.
Advantages include:
- Extremely gentle flow
- Excellent biological filtration
- Inexpensive
- Safe for fry
- Virtually impossible to clog completely
Disadvantages:
- Limited mechanical filtration
- Visible inside the aquarium
- Requires an air pump
Ideal for:
- Fry tanks
- Shrimp aquariums
- Quarantine tanks
- Breeding aquariums
Undergravel Filters
Although less common today, undergravel filters were once extremely popular.
They work by drawing water through the substrate.
Advantages:
- Simple design
- Low maintenance
- Inexpensive
Disadvantages:
- Difficult to clean
- Limited filtration capacity
- Unsuitable for planted aquariums
Most modern aquariums now use internal or external filtration instead.
Sump Filtration
A sump is a separate filtration system located beneath the aquarium.
Water overflows into the sump before being pumped back into the display tank.
Advantages:
- Huge media capacity
- Excellent oxygenation
- Easy equipment concealment
- Highly customisable
- Ideal for advanced systems
Disadvantages:
- Expensive
- Complex installation
- Requires drilled aquariums or overflow systems
Sumps are the preferred filtration method for most reef aquariums.
Which Filter Is Best for Freshwater Aquariums?
For most freshwater aquariums:
Small Aquariums (Under 60 Litres)
Recommended:
- Internal filter
- Hang-on-back filter
Medium Aquariums (60–200 Litres)
Recommended:
- External canister filter
These provide excellent biological filtration while keeping equipment out of sight.
Large Aquariums (200 Litres and Above)
Recommended:
- Large external filters
- Twin external filters
Using two filters provides redundancy and increased biological capacity.
Which Filter Is Best for Goldfish?
Goldfish produce significantly more waste than tropical fish.
The best options include:
- Large external filters
- Oversized internal filters
- Twin filtration systems
Aim for filtration capable of circulating the aquarium volume 5–10 times per hour.
Which Filter Is Best for Planted Aquariums?
Planted aquariums benefit from:
- External filters
- Gentle flow
- Large biological capacity
External filters also allow inline CO₂ equipment and UV sterilisers to be installed neatly.
Which Filter Is Best for Shrimp Tanks?
Shrimp require gentle water movement.
Ideal choices include:
- Sponge filters
- Small hang-on-back filters with intake guards
- Gentle internal filters
Always use a sponge pre-filter to prevent baby shrimp being drawn into the filter.
Which Filter Is Best for Marine Aquariums?
Marine fish-only aquariums often use:
- External filters
- Sumps
Reef aquariums almost always benefit from:
- Sump filtration
- Protein skimmers
- Return pumps
- Biological media
- Reactors
How Much Water Flow Do You Need?
A useful guideline is filter turnover.
Tropical Aquariums
Aim for:
4–6 times the aquarium volume per hour
Goldfish
Aim for:
5–10 times per hour
Planted Aquariums
Aim for:
5–8 times per hour
Reef Aquariums
Combined filtration and circulation often exceeds:
20–50 times per hour
using additional wave makers.
What Is Filter Media?
Filter media refers to the materials placed inside your filter.
Different media perform different jobs.
Mechanical Media
Examples:
- Filter sponge
- Filter floss
- Fine polishing pads
Removes visible debris.
Biological Media
Examples:
- Ceramic rings
- Bio balls
- Sintered glass
- Porous rock
Provides a home for beneficial bacteria.
Chemical Media
Examples:
- Activated carbon
- Phosphate remover
- Purigen
- Zeolite
Removes dissolved impurities.
How Often Should You Clean Your Filter?
Most filters require cleaning every 4–8 weeks, depending on stocking levels.
Never wait until water flow has reduced dramatically.
Cleaning involves:
- Rinsing sponges in old aquarium water
- Cleaning impellers
- Removing accumulated debris
Never wash biological media under untreated tap water.
Should You Replace Filter Media?
Not all filter media needs replacing.
Sponges
Usually last several years.
Replace only when physically damaged.
Ceramic Media
Often lasts for many years.
Replace gradually if necessary.
Carbon
Replace every few weeks if being actively used.
Filter Floss
Replace when clogged.
Never replace all biological media at the same time.
Can You Have Too Much Filtration?
Generally, no.
Most aquariums benefit from oversized filtration.
However, excessive water flow may stress:
- Bettas
- Fancy Goldfish
- Shrimp
- Long-finned fish
If necessary, use spray bars or flow diffusers to soften the current.
Should the Filter Run All the Time?
Yes.
Filters should operate 24 hours a day.
Beneficial bacteria require:
- Oxygen
- Water flow
- A constant food source
Turning the filter off for extended periods can damage the bacterial colony.
Common Filter Problems
Reduced Water Flow
Usually caused by:
- Dirty sponges
- Blocked hoses
- Dirty impeller
Regular maintenance usually solves the problem.
Noisy Filter
Possible causes include:
- Air trapped inside
- Dirty impeller
- Worn bearings
- Loose components
Cleaning often restores quiet operation.
Cloudy Water
Often caused by:
- New aquarium bacterial bloom
- Dirty mechanical media
- Overfeeding
Regular maintenance and patience usually resolve the issue.
Choosing the Right Filter Brand
Several manufacturers produce excellent aquarium filters.
Popular brands include:
- Fluval
- Oase
- Eheim
- Aquael
- JBL
- Sicce
- SuperFish
- Tetra
- Interpet
Each offers models suitable for different aquarium sizes and budgets.
Common Beginner Mistakes
Buying a Filter That’s Too Small
Always choose a filter rated for at least your aquarium size.
Slightly oversized filtration is often beneficial.
Cleaning the Filter Too Thoroughly
Beneficial bacteria live inside the filter.
Avoid over-cleaning biological media.
Replacing All Media Together
This removes much of the biological filtration and may trigger a mini cycle.
Replace media gradually.
Switching the Filter Off
Filters should never be switched off overnight.
Continuous operation keeps beneficial bacteria healthy.
Final Thoughts
Choosing the right aquarium filter is one of the most important decisions you’ll make when setting up a fish tank. The best filter is one that provides reliable mechanical, biological and, where necessary, chemical filtration while matching the size and needs of your aquarium.
For most freshwater aquariums, external canister filters offer the best combination of performance, media capacity and ease of maintenance. Internal filters remain an excellent choice for smaller aquariums, while sponge filters are ideal for breeding tanks and shrimp aquariums. Marine and reef systems often benefit from sump filtration for maximum flexibility and filtration capacity.
At Charterhouse Aquatics, we stock one of the UK’s widest ranges of aquarium filters and filter media, including internal filters, external canister filters, sponge filters, hang-on-back filters, sump equipment, biological media, filter sponges and replacement parts from leading brands such as Fluval, Oase, Eheim, Aquael, JBL, Sicce, SuperFish and many more. Whether you’re setting up your very first aquarium or upgrading an existing system, our expert team can help you choose the perfect filtration solution.
