I have an emperor 400 and a 280 on my 90g(as well as an Eheim 2028) and I can't find a water level to get them quiet. I think marineland says to make the water level 1/2" from the lip, but I made a post a while back about how they don't sit flat on the 90's frame, they sit like 1/2" up, and marineland and the consensus here both say that's fine. So the thing is to get the water level equivalent to 1/2" from the flat mounting surface on the filters I would have to fill my tank up all the way to the top, is there anything wrong with doing that? When I've tweaked it around that 1/2" from the tank lip it seemed like there was a point when more water actually made it louder. Aside from that, the 280 has about 1/8" longer ramp. It seems like the 280 is the loudest, my theory being that its basically half of a 400 but it's pumping 140% as much water out that one half of the 400(the ramps are the same width)
