It's hard to say; there are a lot of things that could contribute to your dying HC. But what pops out at me is the lack of CO2. You say that you run CO2 for 6 hours, but you run your lights for 8 hours. Maybe you could up the duration of the CO2 to 8 hours as well? And how many bubbles per second (bps) of CO2 go into your tank? You may want to pick up some Flourish Excel from the store. It's a liquid carbon supplement that makes it easier for HC to photosynthesize. HC really likes CO2, so you may want to try that.