I usually recommend a minimum of 20x turnover, so that means 125 x 20 = 2500gph. IMO 2 powerheads are always better than one, so that means 2 x 1250 gph. Again, this is minimum IMO and a good starting point. It depends how your tank is stocked and how full it is. My tanks run at about 30x-40x turnover. Personal preference also has a lot to do with it.