The phosphate may have an impact, but I doubt your lights are really to blame. I had mediocre coraline growth in my 90 g until I started using lime water (kalkwaser) to replace my evaporated water and the coraline growth just took off. I have to scrape the stuff off the front glass and powerheads constantly now. I'm not sure exactly why the kalkwasser made such a big difference when I was already supplementing calcium, but it is certainly worth trying in your situation.