To me, the drawbacks of using multiple AC110 are:
1. Clutter of intake tubes (there will be 3 intake tubes if I get three AC110's). Basically, 3 intake tubes will look uglier than the 1 intake tube of Fx6.
2. Must have empty space between aquarium and wall so the hang on back filter can fit. Can also mount 2 filters on each side of the 125 gallon tank, but that would look ugly and circulation reduced in center of tank.
3. Noise - 3x AC110 will make more splashing noise than 1x Fx6. The motor noise for either setup is negligible compared to splash noise.
4. Lower biological volume than Fx6. The Fx6 has an impressive biological filter volume of 1.5 gallons. Three AC110's can easily match or exceed this if you replace the carbon bag with a second sponge and also use a prefilter sponge.
The advantages to having 3x AC110 filters are:
1. Improved water circulation within each section of a 125 gallon long tank.
2. Power consumption - Fx6 uses 43 watts versus each AC110 uses 14 watts. So three AC110 will need same power as Fx6, but AC110 setup will have almost TRIPLE the gallons per hour (563 gph
for Fx6 vs 1500 gph
3. Lower chance for prefilter to get clogged due to 3x more mechanical surface area (on prefilter) when using 3 prefilters instead of just 1 prefilter. Less frequent clogs means less maintenance.
4. When one AC110 filter goes out of order, two AC110 filters continue to run and your fish will live. If a single Fx6 goes out of order several years from now, your fish may die within 4 to 6 hours while you are away at work.
5. Increased mechanical surface volume if you place a prefilter sponge (with very coarse pores) on each of the 3 intake tubes. Additionally, food bits that get caught on the prefilter sponge will still be eaten by fish, whereas food that enters filter chamber will just rot and produce ammonia with no benefit to the fish. I highly recommend this prefilter from Amazon - it fits most HOB
and canister intakes using adapters (all included). I have it on my AC50, AC70, AC110. The size specs are wrong on Amazon, it's really 3 inches diameter and 4 inch length for the sponge (not 1x1x1 inch). My only complaint is that the connector pipes are made of brittle plastic that will probably shatter if dropped on the floor:
Amazon.com : Filter-Max III Prefilter : Aquarium Prefilters : Pet Supplies
I have looked long and hard at the Fluval C3/C4 series which basically is a redesigned version of the AquaClears (which Fluval also owns). It has been confirmed by dozens of Amazon verified owners that the C3/C4 impeller chamber is more tightly-fitted than the AquaClears, causing sludge to jam the impeller very often, requiring impeller clean-up every 2 to 3 months, depending on bioload. This can happen to AquaClear filters too, but more like every 16 to 18 months in my experience. For this reason alone, I will stick with AquaClears for now.
If someone can convince me that canister technology is vastly superior to HOB
in terms of ease of maintenance, superior biological/mechanical filtration, or oxygenation, then I would buy two or three Fluval 406 instead of just one Fx6.