I have read a number of articles that says adding any more that 15-20ppm of co2 is harmful to your fish. I have 130watts of light over a 20 gallon tank. Ive kept my co2 at 20ppm's so i dont kill of my fish. yet for better results a number of people have told me with that high of light i should have it at 30-40ppm of co2. Any comments on this issue would be apprieciated?