I run a six light t6 system and have had a few cyano
break outs but I don't believe it's cause from lighting, it's caused from nitrate and phosphate. And if you're testing your water for both of those things you may be getting a reading of 0ppm but it maybe a false reading because the cyano
is actually eating the phosphate so I don't think you will get a true reading. You may want to test te water you're using to do your pwcs before you add and make sure it's phos free. Do you have any pictures??