well for a 6ft tank, DIY CO2 is practically a waste unless you wanted to give say 1 plant in particular more CO2 or something similar.
looking at CO2 setups - a pressurised system is somewhere in the region of $400 all up, plus the cost of the diffuser, which can range from $30 - $200 for one that is suitable for a tank that size.
I won't bother offering you the diffusers I have, cause they won't be much use on a 6ft tank unless you wanted to have it running under 1 plant in particular.
Also, have a think about if you really need CO2 injection. do the means justify the end?