Filtration
Internal filter
Common in starter kits. It works, it is easy to service, and it takes up tank space and is visible. Media capacity is limited and the outflow is usually a single jet that leaves dead spots.
Short answer
A submersible pump-and-sponge unit that sits inside the tank. Simple, cheap and space-efficient in the room but not in the tank.
What it does
- Mechanical and biological filtration
- Directed outflow, often with a venturi air intake
Strengths and where it fits
Strengths
- Inexpensive
- No risk of leaking outside the tank
- Easy to remove and rinse
- Often includes an adjustable venturi for aeration
Good fit for
- Small tanks
- Quarantine tanks
- Supplementary flow in a larger tank
Limitations and where it is the wrong choice
Limitations
- Occupies tank space
- Visible
- Small media volume
- Single-jet outflow creates dead spots
Poor fit for
- Display aquascapes
- Large or heavily stocked tanks