With that large a tank you are in a different ballpark when it comes to getting enough CO2 in the water. A simple diffuser won't do it. A single external DIY reactor probably won't do it. And, getting enough water circulation all over the tank is essential to get the CO2 to all of the plants in the tank. I agree with Stevie that you can learn a lot by reading lots of old threads here.
But, another option is to go for low light intensity, so you aren't driving the plants to need a lot of CO2.