DIY C02 on a tank that size will likely cause you grief unless you lower the light intensity a bit more, the WPG rule is iffy on large tanks, so some specific specs may help a little more.
I doubt you will get diffusers to work well if at all with DIY C02.
You're intended dosing is a bit excess based on the tank specs listed.
You will need some Trace minerals, Plantex, Flourish Comprehensive or Tropica plant nutrition, I recommend the Plantex, it is cheap and just as effective as the others, save up for a pressurized C02 system.
Do you have any fauna?
Do you have a lot of plants?
Filtration? (very important)
Substrate?
Exactly what kind of light? bulbs? wattage? do you have.
Based on you're listed information I would recommend lowering light intensity a bit and use the DIY C02 and see how things go with a dose of 1Tsp K2S04 and 15ml trace once a week for starters.,
If you start to notice BBA forming (pay attention, it is hard to get rid of), stop DIY C02 and do a heavy water change and leave the light off for a couple days.
Need more info.
Go Pressurized or buy stock in Tylenol.
