Internal filter

An internal filter sits inside the aquarium, driven by a submerged motor. It is the default choice for small tanks, quarantine, hospital tanks, and breeding setups where external plumbing is impractical.

How it works

A pump draws water through mechanical sponge (and sometimes carbon or ceramic media), then returns it via an outlet nozzle. Most models include an air venturi that adds surface agitation for oxygenation.

Sponge intake ──► pump ──► outlet nozzle ──► tank
       │
       └── optional carbon / ceramic chamber

Types

Style	Features	Best for
Sponge filter + powerhead	Gentle, fry-safe	Breeding, shrimp
Compartment internal	Sponge + carbon + ceramic	General nano tanks
Internal canister-style	Larger media volume	60–150L tanks without cabinet space
Corner filter	Discrete placement	Aesthetic nano setups

Sizing

Aim for 4–6× tank turnover per hour for gentle species; up to 8× for goldfish or heavy bioload.

Tank volume	Typical internal rating
10–20L	100–200 L/hr
30–60L	300–500 L/hr
80–120L	600–800 L/hr (or upgrade to HOB/canister)

Above ~120L, a hang-on-back or canister filter usually offers better media capacity and easier maintenance.

Advantages for special setups

Quarantine / hospital — dedicated filter seeded with biomedia from main tank; remove carbon during medication
Fry and shrimp — sponge pre-filters stop intake from sucking babies
Backup filtration — keep a spare internal ready if the main filter fails
No overflow risk — unlike sumps; simple for beginners

Flow and placement

Adjust outlet — aim along glass to reduce current for bettas and gouramis
Raise outlet — surface ripple improves gas exchange
Never bury in substrate — intake must stay clear
Multiple small filters — sometimes better than one oversized unit on long tanks

Maintenance

Rinse sponge weekly in old tank water — never tap water
Replace carbon monthly if used (remove during meds and plant fertilisation)
Clean impeller every 2–3 months — flow drop is usually impeller gunk
Reseed after antibiotics — beneficial bacteria may be damaged; see fish disease overview

Internal vs other filters

Feature	Internal	HOB	Canister
Media capacity	Low–moderate	Moderate	High
Ease of install	Easiest	Easy	Moderate
Visible in tank	Yes	Minimal	Hidden
Cost	Lowest	Low–moderate	Higher
Best tank size	< 120L	< 200L	Any

Pros & cons

Advantages

Inexpensive and plug-and-play
Ideal for QT, hospital, and nano tanks
No external plumbing or overflow
Gentle options for fry and shrimp

Considerations

Takes display space
Limited media vs external filters
Regular cleaning needed in small volumes
Large models can look bulky in aquascapes

Frequently asked questions

Internal or external filter?

Internals suit smaller tanks, quarantine and hospital setups, and anywhere an external is impractical. Externals hold more media, are easier to service, and free up display space. Below roughly 60 litres the difference matters little; above it, external filtration is usually the better investment.

How often should I clean the media?

When flow drops noticeably, not on a fixed schedule. Rinse mechanical media in old tank water; never under the tap, since chlorine kills the bacteria colony. Biological media should be disturbed as little as possible.

Why did my tank cycle crash after I cleaned the filter?

Almost certainly the media was rinsed in tap water, or too much was replaced at once. The nitrifying bacteria live on that media, and chlorine kills them on contact. Replace media in stages, weeks apart, so the remaining colony can repopulate the new material.