You aren't going to get enough CO2 from a single 2 liter bottle, no matter how you diffuse it. You probably need 4 times that to have a good chance to have enough CO2. The 2 bulb T5HO light will be giving you enough light that good CO2 is essential, so your best bet, by far is pressurized CO2. It costs money up front, but if you take your time searching the swap n shop forum, craigslist, ebay, local aquatic plant clubs, etc. you can usually get a much lower price than you expect. Then you have a good chance to succeed with the tank.