Why Aquarium Water Filters Are Essential
Aquarium water filters are a key component of equipment responsible for water circulation and purification, making it safe for inhabitants. In my experience, a properly selected filter significantly simplifies aquarium maintenance and supports fish health.
Modern filters perform several functions simultaneously. Even the simplest devices provide at least two types of filtration—mechanical and biological—while more advanced models ensure all three types of purification. This allows creating a stable ecosystem in a freshwater aquarium without constant intervention.
Types of Water Filtration
Mechanical Filtration
Mechanical filtration uses sponges, filter bags, and filtering fibers that physically remove debris from water. Such filters effectively trap large particles: leftover food, dead plant matter, and fish waste.
Mechanical filters are especially useful during initial substrate preparation or after gravel cleaning. If the substrate is well-rinsed, the need for intensive mechanical cleaning decreases, but you cannot eliminate it entirely—solid particles still form during inhabitants’ life processes.
Biological Filtration
Biological filtration is the most important water purification process and aquarium ecology component. Through bacteria, organic pollutants such as ammonia and nitrites are broken down and removed.
Nitrifying bacteria Nitrosomonas break down ammonia, converting it to nitrites, while Nitrobacter bacteria transform nitrites into nitrates. This process is called nitrification and forms the foundation of biological balance in an aquarium.
The main working element of a biofilter is a complex of microorganisms inhabiting substrate surfaces, and biofilter colonization requires time. This is why you cannot immediately stock a new aquarium heavily with fish—the beneficial bacteria colony must establish itself first.
Besides bacterial biofiltration, a plant-based variant exists. Aquatic plants actively absorb nitrates and other breakdown products, using them for tissue building. Sometimes a separate planted tank is created for this purpose, or hydroponic systems are used where plant roots are bathed in water from the main aquarium.
Chemical Filtration
Chemical filtration uses activated carbon and other adsorbents to remove dissolved toxins and odors. This type of purification is especially important after treating fish with antibiotics or when you need to remove organic dyes, heavy metals, and other chemical substances from water.
The most common chemical filtration media is activated carbon. It should be used in courses rather than continuously, since carbon becomes saturated over time and stops working.
Internal Aquarium Water Filters
Internal aquarium filters are distinguished by low cost and moderate working efficiency. The device is placed directly in the water. This is the most popular equipment type for beginner aquarists and small tanks.
Sponge Filters
Sponge filters provide mechanical and biological cleaning and are recommended for fry tanks or as a supplementary system. They operate from a compressor via an air-driven “bubble lift” and are safe for young fish that won’t get sucked into the filter.
The most common, inexpensive, and effective filter media is sponge. However, it’s important to use special aquarium sponge rather than household varieties.
Undergravel Filters (False Bottom)
Undergravel constructions consist of a perforated plate placed 1.5–2 cm above the bottom. Substrate is poured over it, becoming the filtering element. Water is pumped from beneath the plate by a powerhead or airlift, passing through the gravel layer.
Such systems are effective but have disadvantages: nutrient leaching from substrate hinders rooted plant growth and may promote algae proliferation. Additionally, aquarium cleaning with an undergravel filter requires more effort.
Classic Internal Filters
Modern internal filters are compact devices with a pump and media compartment. They attach to the aquarium wall with suction cups and provide continuous water circulation. The only significant disadvantage is they don’t always fit well into the design, but this problem is solved by placing the filter behind plants or decorations.
External Aquarium Water Filters
External filters are placed outside the aquarium, which saves internal space and doesn’t spoil the underwater landscape view. They are especially popular for large tanks and densely populated aquariums.
Canister Filters
Canister filters are a variety of external filters with a sealed canister-shaped body. They are distinguished by high productivity and quiet operation, and are often used in professional systems.
Canister filter volume should be approximately one-tenth of the aquarium volume. They are usually placed in the cabinet under the aquarium. In my experience, canister filters are the optimal choice for aquariums from 100 liters: they are silent, efficient, and require maintenance less frequently than internal filters.
The larger and more porous the substrate in the filter, the more beneficial bacteria can live in it, so for aquariums over 100 liters, the choice is clearly in favor of an external filter.
Tubular Filters
Tubular filters consist of a long coiled hose (up to 20 meters). They create conditions for anaerobic bacteria reproduction, completing the cycle of toxic ammonium compound transformation. They are usually used together with other filter types, placed at the output of a composite system.
Mill Wheel
The mill wheel is an extravagant construction with wide blades onto which water falls, causing the wheel to rotate. The blades quickly become colonized by bacteria, and their rough surface increases the useful area for the colony.
Besides biological purification, the device effectively aerates water. Disadvantages include unusual appearance, constant water splashing, and vulnerability—even brief cessation of water supply leads to wheel drying and bacterial death.
How to Choose a Filter: Performance and Power
Filter performance should be at least 3 times greater than the tank volume. For example, for a 20-liter aquarium, performance should be no less than 60 l/h. Manufacturer recommendations correspond to the minimum recommendation of 2-3 volumes per hour.
I recommend aiming for 3–5 volumes per hour for a regular aquarium and up to 6–8 volumes for densely populated tanks. Keep in mind that actual water circulation is 30% less than pump performance due to hydrodynamic resistance.
Filter Media
For maximum effective water purification, the filter must have quality media installed: activated carbon, bio-ceramic, porous sponge, synthetic batting.
Correct media placement order:
- Coarse-pore sponge (mechanical prefiltration)
- Medium-pore sponge
- Bio-ceramic or ceramic rings (biological purification)
- Activated carbon or zeolite (chemical purification, as needed)
- Fine synthetic batting (final mechanical purification)
Ceramic rings are hollow tubes with porous surfaces, whose area is 15 times greater than other media. This makes them an ideal substrate for nitrifying bacteria colonies.
Filter Maintenance and Cleaning
An internal filter in an average aquarium is usually cleaned once a week, while an external filter is cleaned once a month. However, these timeframes are approximate and depend on specific conditions.
When it’s time to clean the filter, the flow will tell you: if it has become weak, it’s time to clean. This is the most reliable indicator of clogging.
Important cleaning rules:
- When rinsing the sponge, it’s advisable to use water drained from the aquarium rather than tap water to preserve part of the microflora
- If you clean the filter completely, the aquarium will be without biological filtration until the bacterial population recovers, so only part of the material is cleaned, approximately 1/3
- Don’t use hot water or detergents—they destroy beneficial bacteria
- Don’t perform filter cleaning during water changes or when adding new inhabitants
With proper maintenance, a filter works for years, providing stable conditions for fish and plants. If you need to clean aquarium walls, do it separately from filter cleaning to avoid creating unnecessary stress for the biosystem.
Pumps and Aeration
Powerheads or “bubble lifts” are used to ensure water flow through the filter. Powerheads are generally quieter and more productive. The “bubble lift” operates from an aquarium compressor and, besides water movement, provides additional aeration, which can be important for some fish species.
The choice between these systems depends on the type of aquarium and equipment. For quiet bedrooms and children’s rooms, powerheads are preferable; for aquariums with high population density, airlifts work well.
Conclusion
Aquarium water filters are not just a technical device but the foundation of a healthy ecosystem. A properly selected and maintained filter allows creating stable conditions, minimizes the need for frequent water changes, and ensures comfort for your pets. In my experience, investing in a quality filter pays off through fish health and simplified aquarium care. Understanding the various types of filtration and proper aquarium technical equipment setup ensures long-term success in fishkeeping.
Article updated: 2026-10-19
