A skimmer works by mixing tiny air bubbles with water, when this happens hydrophobis organics will stick to thses bubbles which will then be forced into the collection cup forming a fould smelling skimmate. Nutrient export is a must, and a skimmer is the most common pice of ewuipment to deal with excess nutrients. Refugiums, water changes, and activated carbon are otehr methods use dto control nutrient export. A combanation of two or more of these works well for most tanks. There are a ton of skimmers out there, I started out with the Lee's counter current skimmer, worked well for me also, but was a pain to keep running with a rapid evaporation rate. A skimmer is one of those things you dont want to skimp on, for a 55 I would suggest a berlin class HO, or a remora pro. Is you have a sump you have wider variety of skimmers to choose from, you could consider a down draft or a larger venturi. Counter currents work well, if properly adjusted. Stay away from new and old sea clones, they do not provide enough air/water contact and work are only big enough fo tanks up to 30 gallons, and even on smaller tanks the skimmate produced is poor. HTH