Filtration is the backbone of any healthy aquarium, as it helps maintain water quality by removing physical debris, breaking down harmful chemicals, and supporting beneficial bacteria. A good filtration system creates a stable environment where fish, plants, and invertebrates can thrive.

There are three distinct filtration methods, and dozens of filter styles. We'll cover the most common ones here.

Types of Filtration:

1. Mechanical Filtration: Removes solid waste like uneaten food, fish waste, and debris from the water using filter media like sponges, floss, or pads. It keeps the water clear and prevents waste from breaking down into harmful chemicals.
2. Biological Filtration: Uses colonies of beneficial bacteria that live on filter media to convert toxic ammonia (from fish waste) into nitrite and then into safer nitrate through the Nitrogen Cycle. This is essential for maintaining a healthy and balanced aquarium.
3. Chemical Filtration: Involves media like activated carbon, resins, or zeolite to remove dissolved impurities, odors, and toxins from the water. While not always necessary, it can be useful for specific issues, like removing medication after treatment.

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Types of Filters:

Air-driven Sponge: a coarse sponge filter powered by an air pump.

Pros: Cheap and easy to clean and replace.

Cons: Small surface area for bacteria. Low water flowrate.

Recommendations: Excellent for quarantine tanks and tanks with a low bio-load. Not suited for tanks with large bio-load.

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Internal Power Filter: a small internal pump with a basket for sponge and sometimes bio-media

Pros: Decent flow rate, and better at catching debris than a sponge. Models with room for bio-media are more suitable for heavier bio-loads

Cons: Small media baskets require more regular cleaning, and some models require expensive cartridges instead of cheap replacement media.

Recommendations: Great for small to medium tanks, but limited media capacity and frequent cleaning requirements may cause issues.

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Hang on Back (HOB Filter): a filter that hangs from the side of the aquarium.

Pros: HOB filters are external, and don't take up very much space within the aquarium. Excellent surface agitation promotes better oxygen levels in the water. Easy to move around as needed. Easy to clean and maintain. Decent water flow.

Cons: Limited capacity for media, often requiring replacement of special cartridges. Requires high water levels to function, and can limit the use of lids.

Recommendations: Suitable for small to medium tanks. Relatively low flow rate and limited media capacity reduce effectiveness, but the small footprint in the water make it a versatile filter for aquascaped nano tanks.

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Undergravel Filter: a grate (usually plastic) under the gravel, with PVC uplifts. Can be powered by an air-pump or powerhead.

Pros: Uses gravel bed as biological media, providing unparalleled biological filtration for an aquarium. Driven by air-pumps or powerheads. Takes up little space in the aquarium. Extremely cost effective

Cons: Build-up of organic matter under the gravel bed may lead to ammonia being released when gravel is disturbed. Cannot be used with sand or soil.

Recommendations: steer clear of undergravel filtration. The risk of ammonia build-up under the gravel is far too high, unless gravel is vacuumed frequently (50% vacuumed each week).

External Canister Filter: external filter with a large canister typically filled with baskets of sponge and biomedia.

Pros: Large baskets for exceptional media capacity. Customisable filter inlets/outlets and versatile placement allow for an myriad uses in the aquarium. Comes in multiple sizes and flow rates for multiple purposes. Some models and brands come within-built UV filtration and some may also come with heaters (or space inside for one), and even digital readouts and WiFi/Bluetooth connectivity for ultimate control.

Cons: Can be quite expensive, difficult to clean and cheaper models may be rather noisy or inefficient. Takes up extra space outside the aquarium, and risks leaking.

Recommendations: Canister filters are our recommended filtration method due to their versatility and filtration capacity. Suitable for small to extra large aquariums (multiple canisters recommended for tanks above 500L). Not suitable for low-budgets, as cheapem odels may be less efficient or noisy.

Sump Filtration: an external sump (usually another fish tank) that contains filtration media and is plumbed directly into the aquarium.

Pros: Extremely large capacity for filtration media, extremely customisable and provides a great place to tuck unsightly equipment like submersible heaters and thermometers. Plumbing can be rigged to be unobtrusive and auto-top up units can be mounted to limit effects of evaporation. Complete units can be purchased that come pre-plumbed and ready to set up.

Cons: requires a large amount of external space, and can be quite complex to operate. Can become clogged and overflow easily, or stop working if there is too much evaporation.

Recommendations: Sump filtration is easily the most efficient and effective filtration method todate. With a huge capacity for filtration media, and a discreet place to dump all that unsightly in-tank equipment, it's easy to understand why people love this type of filtration. However, the complexity involved with setting up and maintaining a sump filter makes it a poor choice for beginners, and entirely unnecessary for small or medium tanks.

Surface Skimmer: a small internal power filter that draws water down through the filter, pulling debris from the surface of the aquarium.

Pros: Small, unobtrusive and great for surface agitation

Cons: minimal capacity for filter media. (Usually) low flow rate

Recommendations: Skimmers are perfect for removing floating debris (like the much hated Duckweed), and for promoting surface agitation. They should be used in concert with larger filters, and are definitely recommended for heavily planted tanks or tanks with injected CO2. You can often buy surface skimmer attachments for sump or canister filters.

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No Filter:
Pros: Nothing to clean, or maintain

Cons: No way to remove physical debris from the aquarium, no housing for biological media,

Recommendation: Please use a filter. Unless you have an advanced understanding of natural filtration methods like the Walstad method (which relies on plants and substrate to maintain balance), a no-filter setup poses significant risks to your fish. Without proper water movement or waste processing, ammonia and toxins can quickly build up, endangering aquatic life.

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