rule of thumb (not mine) but mentioned in about.com about marine aquariums, is 10-20 times the water volume in flow. For examle...if you have a 10g tank, then you would want one powerhead that pushs 100g per hour. this gives you a 10 time an hour turnover rate. if the powerhead does 200g per hour then your pushing 20 times an hour turnover. you can get this easily with 2 powerheads. for your 55g tank you want a total of 1100gph turnover. a single k2 does 600gph, so (2) k2's gives you 1200gph or a 20 time turnover. and (2) k3 will give you 1700gph turnover either way you will be really good to go.