i am facing the same question right now in my 55 gal i have 1korrila #4 and a rio 600. i am going to take the rio out and put another korrila in just dont know what size im going to add yet,right now thinking of the #3 but still undecided
in a 46 gal bow front i would prolly concider going with 2 korrila #3's in, ultmitely its up to you and what you prefer and how much flow you want and what kind of critters you keep in your tank and what they like