If you use DIY CO2, using Excel helps to prevent BBA infestations. But, with pressurized CO2 it really serves no purpose. But, I don't see it doing any harm either.
I am running both at the moment and based on what I've read in the following thread the Excel is there to combat BBA. I am though now cutting back on it to try and preserve my vals. http://www.plantedtank.net/forums/showthread.php?t=20172
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