Red algaes like lower lighting. At one time I ran 2X96w PC and left one side alone until it was completely covered in a deep red algae. And then I released a turbo in my tank (I still ask myself "why?!) and lost all the red in a couple days I've read before that Kelvin ratings are almost always off. With that in mind.... Maybe the T5's aren't matching up to their Kelvin rating where the metal halide is closer to being a true 10K?