🔄Last Updated: 12 March 2026•Originally published: 10 February 2020
A filter is the life support system of any aquarium — without effective filtration, toxic ammonia and nitrite from fish waste accumulate rapidly, causing stress, disease, and death. Choosing the right filter type and size for your tank is the most important equipment decision you’ll make in fishkeeping.
Internal vs External vs HOB
| Type | Best For | Pros | Cons |
|---|---|---|---|
| Internal | Small tanks (under 150L) | Affordable, easy install | Takes up tank space, less media |
| External (Canister) | Medium-large tanks (100L+) | Powerful, huge media capacity | More expensive, needs cabinet |
| Hang-on-Back (HOB) | Small-medium tanks | No tank space used, easy access | Visible, can be noisy |
| Sponge | Fry/shrimp tanks, backup | Gentle flow, cheap, great bio | No chemical filtration, visible |
Quick Comparison: Best Filters UK 2026
| Product | Type | Best For |
|---|---|---|
| Fluval FX6 | External | Large tanks (up to 1500L) / Power |
| Oase BioMaster Thermo 850 | External | Built-in heater / Premium |
| Fluval U3 | Internal | Medium tanks (up to 150L) |
| Seachem Tidal 55 | HOB | Quiet operation / Custom media |
| Eheim Professional 3 | External | Reliability / Longevity |
Sizing Rule
Your filter should process at least 4× your tank volume per hour. A 200-litre tank needs a filter rated for at least 800 litres/hour. For heavily stocked tanks, planted tanks with CO2, or messy fish (goldfish, cichlids, plecos), aim for 6-10× turnover.
The Three Types of Filtration
- Mechanical: Sponges and pads trap solid debris. Clean (never replace) in old tank water to preserve bacteria.
- Biological: Ceramic rings, bio-balls, or sintered glass provide surface area for beneficial bacteria that convert toxic ammonia → nitrite → nitrate. This is the most important filtration stage.
- Chemical: Activated carbon removes dissolved organics, tannins, and medication. Not always necessary — many experienced fishkeepers skip carbon entirely.