I like to have 2 smaller filters then one big ones. For a 55G I would use 2 Aquaclear 70's. The 110 is huge and I would think that its to much waterflow in one area. Just my opinion. I will be using my 110 for my upcomming 85 gallon tank. Lets see how it holds up.
I have mine working in a 125 alongside a magnum 350, a cascade 1000, and a sponge rated to 40 gallons. I'm not worried about it in it's current application but I was thinking about swapping it over to a 75.
I have 2 on my 180g along with a xp3. The sponges in the ac110s are cleaned in hot tap water on a weekly basis, they work very well for mechanical filtration. The biomax it rinsed in tank water, to preserve any bacteria that has colonized it.
I like the AC line but would not use it as a solitary filter on any tank.
Tap water kills the "good" bacteria, no matter what the temp. The sponges in my AC110s are for mechanical filtration and I want to remove as much undisolve particulate watse as possible. The "good" bacteria has more then enough area to colonize with a bag of biomax in each AC and the 2 liters of biomedia in the xp3.