in a perfect world your stitches would be perfect top and bottom, most of the time it is not such a perfect world. one way to make them not so noticable is to use the same thread top and bottom; the only other way is to adjust the tension, until you have the top and bottom tension balanced. if your bobbin thread is pulling up to the top your top tension is too tight. you need to loosen it a little. make a practice sandwich and adjust the top tension about a 1/4 turn at a time, stitch a line, check it, turn the tension dial a little bit more, and keep checking until you have the tension equal to the bobbin tension. if it gets worse you are turning the dial the wrong way. if after a few turns it is not making a difference you may need to tighten the bobbin tension to better match the top. it is always better to adjust from the top than from the bottom when possible