The built in air intake tube on the Skilter is very noisy and it slightly reduces the water flow rate. It's much better if you use an air pump and air stone in the skimmer tube. Just take off the skimmer top and tube, run the airline from an air pump down next to the filter intake and into the bottom of the skimmer tube through one of the slits. The airstone will provide MUCH more air and the whole thing will run better. You'll have higher water flow through the filter, more air in the skimmer and it'll be much less noisy. It's an easy mod to do.
I love Skilters by the way. I had 2 reefs up for years using them and never had a problem with them. They're by far the best HOB
filters for a mini reef, though a 55 is larger than I would use them on. A better bet would be a sump. I hope this helps.