The area above the impeller is "open" to allow one to pull the impeller housing out of the filter for maintenance. That's my guess.
As for the splash back and force from a single water fall, you can divert the flow by using a piece of plastic that is found with the glass canopy tops. That's the strip that attaches to the back pane of glass. It's plastic and meant to be cut to allow for filters and heaters to fit. Anyhow, measure the width of the waterfall section of the AC110 (this can be applied to different HOB
filters) and cut the plastic strip to match the width. Next, take the part that would attach to the glass pane and attach it to the a HOB
. You may need to use some pliers to crimp the "grippy" part for a snug fit. You can heat the plastic using hot water and bend it to a curve that best suite your needs (flow along the top, flow at a 45 degree angle down, etc). If find that the flow along to top is quieter and less disruptive to the fish, plants, and substrate.
I posted a thread on this some time ago and of I find it I will post the link.
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