Even though the manufacturer claims the XP3 can filter up to a 170 gallon tank, it really can't, unless the tank is very lightly stocked. The 350 gallon flow rate almost gives you a 3 times turnover of water volume which is kind of low. Add to that the fact that the flow rate will drop off considerably as the filter media gets dirty and you may not even be turning over 2 times the volume. So unless you are planning to stock your tank very lightly, your single XP3 filter will not provide enough filtration for a 125 gallon tank.