Most common mistake people make with auto's is switching to bloom nutes as soon as sex is shown. Don't switch to flowering nutrients until after the stretch has stopped (week 5-7) if you want to get the most out of the plant. Be VERY careful feeding auto's. They are VERY nutrient sensitive and burn easily. Even the larger auto's that I run can't be pushed past 1.25ec / 635ppm in full bloom without tip burn.
I use General Hydroponics FloraDuo and add Liquid Koolbloom in flower. Killing it so far. Koolbloom doubled my yield.
Can be used in Hydro, Soil, and Soilless.
Check out the links in my Sig for some pretty killer auto grows.
Peace.
Tav