If you do happen to go with an AC 20/30 you could add a sponge to the intake acting as a prefilter. Then, with some creative cutting, take a sponge that is the width of the output, and 1/4" or so thicker than the distance between the inside wall of the tank and the back of outlet. Cut out one corner of the spunge so it looks similar to a stair step. Squeeze the top step between the wall and filter allowing the outflow to flow onto the other step going into the sponge before the tank and reducing the agitation caused by the waterfall effect of the HOB.
As, I see in your first post you have a HOB filter. You could try adding the sponges to that filter and see how it works out.