Well, if we are just working with what you have: first I'd move that heater way over to the corner. Second, I would cluster your plants in groups of single kinds instead of just having them spread all over. I would choose your largest plant and try to make that your focal point (right now the heater is your focal point).
I would also find some rocks and/or wood to make a strong hardscape. Wood might be easier to work with in your tall tank.
6 hours with a single T8 bulb? How about trying to increase by 1/2 hour a week to about 10 hours a day? Stop if you get concerned about too much light causing algae but I suspect you won't have happy plants until they are getting at least 8 hours a day. And a timer is an excellent idea.