Lots of tradeoffs - either will do the job fine.
I have a C3 on my 29 gallon and am also debating C series versus Aqua Clear on my 38 gallon which is currently running an aqua-tech.
First, it helps to understand how each works. Despite the fact they are owned by Hagen/Fluval ... the only parts they seem to share based on the parts list are the motors.
Instruction manuals for both can be downloaded at the Hagen Website ..... which show diagrams and part numbers for both:
http://usa.hagen.com/File/ad1fd135-d772-4217-8203-bf1a7eeeebcb
http://ca-en.hagen.com/File/ede567c9-42ac-4895-b81e-f032a71517e6
Note that the C series uses separate "chambers" for the various stages of filtration while the AC's just stack the media on top of each other.
They both claim to be good for 70 gallons but their filter rates differ. 300gph for AC70 ..... 264 for the C4. I believe both are equally effective. I have heard two explanations for this including that one is measured based on outflow after the media while the other is based on inflow without media.
The other and more likely explanation (based on my experience) is that the difference is the 3 stage vs 5 stage filtration. The first "media" in the the AC series is a sponge pad that can be re-used. The first media in the C series is a poly/foam pad that provides two stage filtration with a more "coarse" layer to capture large debries and then a polishing pad (100 microns or less) to capture very fine debris, It captures such fine debris ... it cannot be re-used effectively because that debris is so "packed in," Thus the media needs to be replaced, not re-used. See my post with pictures about this here:
http://www.aquariumadvice.com/forum...ot-reusable-sharing-my-experience-280070.html
Nest, the C series uses a drip tray which provides additional filtration. I have read (don't know if it's correct) that the water that flows through the drip tray is not counted as part of the flow rate.
Just google youtube videos of both and you'll see what I mean. There are a ton of them.
No matter what filter you buy, you run about a 25% risk of this:
http://www.aquariumadvice.com/forum...alc3-are-enough-best-quiet-filter-276431.html
Which turns out to be a small setback because mine now runs silent .... except for the annooying sound of water splashing unless you really fill the aquarium to the brim. Not sure if the aqua-clear has this "waterfall" problem or not.
Next ....... you want to use a sponge pre-filter. Fluval makes one for the C series which, according to reviews on amazon, also fits the AC.
I hope someone can verify this.
Next, the Fluval C3 intake tube does not go the anywhere near the bottom of the tank. Not sure about the C4 or the aquaclears. As you can see from the instuctions, the spare parts list shows a tube extension for the C series ... but not for the aquaclear. Yet I read somewhere that the instructions to the Aqua-Clear are obsolete and the extension now comes standard on the AC 30, 50, & 70 series.
Can someone chime in here?
You can achieve crystal clear polished water with either one. The C series does it right out of the box. (Although I recommend purigen & floss in lieu of carbon) and in the AC, the "polishing" is best accomplised by stuffing the space for carbon with floss and/or 100 micron polishing pad and/or Purigen. My understanding is that the AC series has more extra room for extra biomedia
that the C series does.
I'd would be nice to hear other observations from people with either or both series of filters.
Note also: The C4 and the AC70 need more space between tank and wall than most filters. About 4 inches.
