I agree with Craigthor; for a tank your size, DIY CO2 is really not an option. You will want to go pressurized CO2, or skip out on the CO2 completely and go for a low light, low tech tank.
You are not seeing pearling because you have insufficient CO2.
The way you are diffusing CO2 right now is fine, but again, due to the low amount of CO2, it is insufficient to meet the needs of the plants.
As Craigthor also mentioned, for fertilizers, I would recommend you use dry fertilizers as well as the EI style of dosing. At the very least, you will need potassium nitrate, potassium dihydrogen phosphate as well as a decent trace mix. Depending on your water chemistry, you may need magnesium sulfate, potassium sulfate and/or calcium sulfate.
Since you said you have 2 WPG over your 75g, I am assuming it is not a stock light, and at the bulbs are probably T8 or better. With this kind of light, you will need fertilization.