#1 I think you meant that all 'normal' sized co2 tanks (i.e. 5# or more) are about equal per # to refill, but it reads like 20oz tanks are included, which is clearly false as they are very cost ineffective.
This has been fixed. I added in a line saying that smaller tanks are essentially cost ineffective due to the cost/pound of gas.
#2 You mention a diffuser as being recommended (i.e. not necessary) but then go on to describe a diffusion method as essential. This is a bit confusing. You might want to discuss different diffusion strategies/devices any one of which is necessary - just sticking a co2 line in the water won't cut it.
I'd like to think of them as two separate ideas. A diffuser is recommended (but not necessary), while diffusion of the CO2 tank (regardless of how you want to diffuse it) is essential.
Some people opt to use an inline reactor, or even sticking the CO2 line near their filter intake, both of which are diffusion methods, but not a diffuser, per se.