wood filler is specific to the stage or condition your floorings in. If your sanding and finishing you would use a hard filler. If your recoating you would use a soft colored filler. What are you doing with your hardwood floor?
If your sanding the old finish off and recoating you can use a few trowelable fillers. Woodwise is what we normally use. You can purchase the color of your species of wood. After sanding your 1st complete pass you can flat trowel all areas needing filling. The filler is pre-mixed. Follow instructions and let dry. The final sand will turn your floor anew if sanded and filled properly. Woodwise is good for both waterbase and omu.