I did a poll a while back and the aquaclears are the most popular out of all the commercial hob filters here on this forum.
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Yes. The Hagen AquaClear has been around for 30 years plus:
http://ca-en.hagen.com/aquaclear
and since 1978 has long been recognized as one of the best HOB filters. The Hagen Fluval C series has only been around about 3 years. So of course, more people are going to own AquaClears.
The AquaClear is Hagen's automotive equivalent to GM's Chevy or Toyota's Camry .... both popular and maybe the best cars for the money but not necessarily the best car. But The Fluval is Hagen's Cadillac or Lexus. I own both working sided by side and for the reasons I stated previously, the Fluval is by far the hands down winner for crystal clear polished water, zero bypass filtration, and ease of using custom media. The Fluval C is the first HOB to give canister filter quality water right out of the box with its polishing pad and drip tray.
Fortunately, the price gap between the AquaClear and the Fluval C is not typically significant. Sometimes, when on sale, the C will be less than the AquaClear.
Pricing today on Amazon is as follows:
AquaClear 50: $38.33
:Fluval C3 $39.99
AquaClear 70: $49.94
Fluval C4: $54.99
Note 1 - both Amazon & Drs F&S change pricing on these items constantly.
I purchased both my C3's from Amazon. My C4 came from Drs. Foster & Smith who ..... on the day I ordered ..... was $5 cheaper than Amazon.
Note 2 - On both the AquaClear 50 and the Fluval C3, you have to order the extension tube to get to the bottom of a typical tank. Once you shell out the extra $7 or $8 for a 69 cent piece of plastic, you might as well spring for an upgraded C4 or AC70 if you have enough space behind the back of your tank. The flow on the C4 can be adjusted downwards from 264 GPH to 159 GPH so you don't have to worry about excess current. Also, when you slow the flow, you decrease the amount of water intake but the water being filtered remains relatively constant as the water is re-filtered multiple times.
To those who have had good luck with other filters, the OP asked what makes one filter better than the other? Not which is the most popular. I sure wish somebody would have cleared this up for me before I threw away $40 and a lot of wasted time on my three week old aquaclear 50 ........ which I am going to retire as soon as my newest Fluval C3 builds up enough BB. [Not enough room for a C4 behind my 38 gallon tank so it's two C3's. I have one C4 on my 29 gallon tank and it's doing a great job running at about 75% of "full throttle."
Edit: For those who want uber crystal clear polished water, go here and see my post #2
http://www.aquariumadvice.com/forums/f60/optimal-filtration-293808.html