The problem could be cause by fluxuations in your fert levels. You shouldn't start ferts or co2, then stop, then start again. It's a gateway to algae and health issues for your plants. I use the Pferts and in honesty, the directions are more geared for high light with co2. I dose the NPK every Monday and trace every wednesday. I do my water changes at 40-50% once a month or so and top off water once a week, on Mondays followed by the NPK. I'm also only running about 1.8wpg of t8 on my tanks. Keep it steady, don't overdose and be patient.