Nice write up!
#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.
#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.