Fluval U3 Underwater Filter

The Fluval U3 is an internal filter from Fluval’s U Series. It sits fully inside the tank and works in freshwater, saltwater and reptile setups up to 40 gallons. Use it as the main filter on a smaller tank or as a second filter on a larger one.

Mounting options

Mount it upright against the glass to push a current along the tank, or lay it horizontally. Horizontal works for shallow tanks, turtle tanks and paludariums with low water, and it can create a small waterfall feature.

Media cartridge

Fluval redesigned the cartridge to trap more debris. A flip-top lid gives quick access, so you can rinse or change the media without pulling the whole filter apart.

Flow control

An easy-grip control panel adjusts flow. Turn it down for bettas and slow swimmers, or up for a busy community or messy turtle.

Why choose an internal filter

• Fits tanks where a hang-on filter can’t hang, like rimless or tight-lid setups.
• Works at low water levels.
• Doesn’t need space behind the tank.

Warranty

Fluval covers U Series filters for 3 years against defects in normal aquarium use.

Maintenance

Rinse the foam in tank water when flow drops. Check the impeller for debris during the same cleaning.

Place it near the surface if you want more surface movement and gas exchange, or lower down in a planted tank where you want to keep carbon dioxide in. Use the flow control to keep the outflow from blasting plants or blowing light fish around the tank.

Alternatives

For a smaller tank, the Tetra Whisper 170 internal filter is a budget option. For a hang-on, see the AquaClear 110.