This really is an old issue even in this forum: From 2008:
Hopefully this information will help people debating whether to go with the Fluval C series vs AquaClear. To sum it up:
1) The AquaClear is prone to the aforementioned bypass issue. Due in part to the fact that water flows bottom meaning less surface area and water going at a higher velocity than most filters (inclucing Fluval C series flow from back to front.
2) The AquaClear does not polish the water out of the box. Inserting tightly packed floss/polishing pad to polish water makes it even more prone to bypass.
3) The Fluval "factory media" contains a pad that goes from coarse to very fine micron level that does polish the water.
4) The AquaClear does not have the drip tray feature that gives more biological filtration.
Some think that the AquaClear is more customizable which simply is not the case. As a matter of fact, I'd say it's the reverse. The only "factory" media I have in mine is the CNodes (essentially the same as AquaClear bio-max). On both my C4 and C3 my media goes Acurel debris reducing pads (cut to fit) -> AquaticLife 100 micron non-woven filter (polishing) pad (cut to fit) -> two 100ml biobags of Seachem Purigen [the C4 has plenty of room so it also has Seachem Matrix in a biobag for extra bio filtration] -> Cnodes. The C series has zero bypass for mechanical filtration (but some bypass for chemical filtration) and zero bypass of biological filtration. There is an indicator to tell you when flow is restricted.
Other interesting observations from watching the C3 and the AC
50 working together side by side:
1) Both filters are rated for tanks "up to 50 gallons" by Hagen. Yet the advertised flow rate is 200 GPH
for the AC
50 and 153 for the C3. Yet, the stream of water coming out of both looks almost identical. As a matter of fact, looking from the bottom of the tank up, it appears more water is flowing out of the C3. (This may be because the flow is a bit restricted due to a 300 micron polishing pad and tightly packed matrix and factory bio media). The AC50 cannot handle a 100 micron polishing pad like the C3.
2) The lip of the AC50 is higher out of the water than the C3. This mean the tank really has to be FULL to avoid those annoying little air bubbles and splashing noises.
I guess I should start a new thread on this that is on point so people trying to make the choice between AC
& C series can make an informed decision.