I am fairly new to this but I have learned that too much light can really throw everything out of balance quickly. Shorten the lighting period in addition to the using fewer tubes per previous commentors.
A diy CO2 yeast reactor running on a tank is better than excel alone with higher lighting I think. Pressurized is absolutely the way to go if you have the money burning a hole in your pocket. I'm happy with my pressurized set up.
You asked about Magnesium sulphate. This is just plain old Epsom salts, available at you local grocery store.