I use a 35 gal sump on my 180 with a 1200 gph turnover. Pump I use is rated right around 1500gph at 0' head and a little over 1200 gph at 4'. I feel that I should add 1 more 600 gph overflow and add additional pump strength to get better turnover on this tank. 55 gal sump would work fine, but I would recommend shooting for 1600-1800 gph which is where I want to get mine for better results. Best of luck.