Oh, its looking so much better! :happy2:
+1 to L. aromatica, but only because I have some from Neon as well. She has some awesome plants.
You might need a bigger diffuser and might be able to get away with one. I contemplated two for my 6 ft tank but have since gotten a larger diffuser from Alan (larger surface area on the frit) and it is doing a great job. It is just in front of the filter outflow. Plants on the other end of the tank are pearling and the pH is stable at each end so I know there is good distribution of gas.
You want to aim for 25-35ppm of CO2 and to achieve that you will likely need to aim for at least 3 drops per second to achieve a good concentration, depending on how efficient your diffuser is.
What's your photoperiod? I would recommend 6 hours to start, especially with so few plants. They need time to adapt to the new light and CO2. If you can, I would jam in some temporary fast growing plants as nutrient sponges. That could be one of the best things you can do to get it all in equilibrium.