Can I just say that I am super sad right now? In all my hours and hours of research, this thread is the first mention I have heard regarding the inefficacy of T12 bulbs. I thought that the numbers indicated a base type, not a glass thickness. I didn't think that that number was even important. Now, I have a 12 sq ft. box with eight t12 40 watt, 6500k bulbs in four ballasts. I put over 60 bucks into this lighting and now I find out it's crap. No wonder my seedlings are growing so slowly.
Can I continue to use the t12's with slowed growth, or do I need to find new lighting? Why is this not something people talk about? People talk about k ratings, wattage, lumens....but not glass thickness.