The Fluval C-Series Clip-On Power Filter is Fluval's five-stage clip-on power filter, sold in two sizes, which hangs on the back rim and runs more stages than a typical hang-on unit.

Key Features

• Five stages rather than the usual three: two mechanical, one chemical and two biological
• The second biological stage is a trickle chamber where water runs over media in the air. Bacteria get far more oxygen there than they do submerged, which is why a trickle stage punches above its size
• Re-filtration chamber lengthens the time water spends in contact with the media
• Two sizes, C2 and C4, for progressively larger tanks
• Media stages are reachable from the top without dismantling the unit
• For use in both freshwater and saltwater aquariums
• Made by Fluval, the aquatics brand of Rolf C. Hagen, which has been building aquarium filters since 1975

Setting It Up

• Hang it on the back rim, prime the chamber with tank water and switch on
• A new tank has no bacteria in it. Cycling — running the filter for several weeks until it processes ammonia and nitrite to zero — has to happen before fish go in, or they sit in their own waste while the bacteria catch up
• Keep the tank topped up. A dropping water level is what makes a hang-on filter noisy
• No filter is silent. Most of the noise people complain about is air trapped in the impeller housing or a unit that is not sitting level

Maintenance

• Change one stage at a time so the bacteria survive the clean
• Rinse mechanical foam in a bucket of old tank water rather than under the tap
• Never wash biological media in tap water and never replace it all at once. Chlorine kills the bacteria living on it, and changing everything together restarts the cycle
• Do not let the trickle chamber dry out — the bacteria in it die within hours if the flow stops
• Unplug anything electrical before putting your hands in the tank

Good To Know

• The trickle stage is the real reason to choose a C-Series over a plain hang-on filter. It gives biological capacity closer to a small canister
• It needs clearance behind the tank, and it is deeper front-to-back than a basic hang-on filter
• The waterfall return is strong. Baffle it or turn it down for bettas and long-finned fish
• Most tanks want 20–30% of the water changed weekly, with the replacement dechlorinated and matched for temperature