Are you using stabilizer? Are you are using an embroidery only machine? I have one but I never change the tension. I find that if this happens is usually due to the thread or the stabilizer because the fabric may be too thin. Try using two layers of stabilizer. If you need to finish something immediately and the stitches look fine, use the same color in the top and bottom. Also, are you embroidering on a large piece of cloth? If it hangs over the sides of the machine it pulls and the bottom thread shows. Make sure the fabric outside the hoop is not pulling.