With a 55 gallon tank I wouldn't buy one that has a high gallon per day, 24 should be plenty per day. http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+4395+12093&pcatid=12093 should be fine for you.
TwistedPenguin;2315508; said:If he used RO water for water changes he'd eventually have 100% RO water in his tank with no minerals whatsoever & a kh of '0' which means no buffering capacity and his tank would crash. The alternative is using all those buffers and quite honestly that's not for the faint of heart, it'd be a pain in the butt. If the water is deemed safe for human consumtion I would bet its fine for fish.