My recommendation is to get to co2 liquid booster. I had a very similar symptom in my very high light tank & couldn't figure it out.. In fact i almost bought even more light, before realizing it was a co2 issue. For $12 bottle, it's a rather cheap diagnostic.
I have a double bright; how large of a tank? I wouldn't have expected a double bright to support much plant life. Not really bright enough.
Love the look of mine though. The shimmer is captivating.