Internal or External Filter: How to Choose the Right Aquarium Filtration
The filter is the heart of the aquarium: it houses the purifying bacteria and keeps the water healthy. A poor filtration choice is the number one cause of failure among beginners. Here's how to make sense of internal filters, external canisters and flow rates, with the price benchmarks observed by Planète Pets.
Internal filter: simple and affordable
The internal filter sits directly inside the tank, usually suction-cupped into a corner. It works well for aquariums of under 100 litres. Its strengths: a modest price (15 to 50 €), instant installation and zero risk of leaks. Its limitations: it takes up space in the tank, its filter media capacity is small and maintenance means putting your hands in the water. Brands such as Tetra, Eheim and JBL offer reliable models in every size.
External canister filter: the benchmark above 100 litres
The external filter is a canister placed beneath the aquarium, connected by intake and return hoses. It offers a large filtration volume, less frequent maintenance (every 4 to 8 weeks) and keeps the inside of the tank clear. Expect 80 to 250 € depending on the volume to be treated. It's the option we recommend for any tank of 100 litres or more, especially if you're aiming for a comfortable beginner volume.
Flow rate: the 3-to-5-times rule
The deciding factor is the actual flow rate, expressed in litres per hour. The classic rule: the filter should turn over 3 to 5 times the tank's volume every hour. For a 120-litre tank, aim for 360 to 600 l/h. Beware: the advertised flow rate is measured with an empty canister; once the filter media is in place, it drops by 20 to 40%. It's better to slightly oversize.
Filter media, in the right order
- Mechanical: foams and filter floss that trap particles.
- Biological: ceramic rings or noodles that house the bacteria converting waste. This is the most important stage.
- Chemical (optional): activated carbon, to be used occasionally after a medication course.
When cleaning, always rinse the biological media in water taken from the aquarium, never under the tap: chlorine would destroy the bacteria on which the nitrogen cycle depends.
Our final advice
Under 100 litres: a good internal filter is enough. Above that: go external without hesitation, even if it means waiting for a sale. And in every case, never switch off the filtration for more than two hours. Browse all our comparisons of aquarium accessories to equip your tank at the right price.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.