I just recently posted pictuers of my tank
http://www.myfishtank.net/forum/fre...on/57537-pictures-my-planted-tank-shrimp.html in the back left you can see a powerhead which is a pump to move water around the tank, i also have a sponge filter on it to help collect junk in the water.
the air hose coming from the co2 bottles is held next to the intake of the powerhead, the sponge does a good job holding it in place.
The yest creat CO2 in the bottles which gets a slight pressure, enough to push past the water pressure and bubble into the powerhead where the impeller chops it up into a fine mist. Also some use there filter in the same way to grind up the co2 bubble to get it to disolve into the water.
Glass diffusers make a finner mist which dissolves better but i'm not sure DIY co2 has enough pseeure with out breaking something to work. Reactors works well at dissolving co2 into the water, better then the filter method but require more work and cost then i think is worht while with a DIY set up.
I would def. use two bottles. You don't want the tube just in the tank, it needs to be misted or something to make it diffuse into the water better. If the tube is jsut sititng in the water the bubble will just float up and not have much surface to vol ratio not giving much change for diffusion. Also for this reason i would have the two bottles hooked up to a tee (which i do) other wise you have two tubes going into the tank and if at either end need two ways to grind up the bubble.
The tube going through the cap into the 2litter bobble needs to be sealed with silicon. The whole system needs to be ai tight or the yest wil not produce CO2 the only place air should get out is the end of the tube going into the tank.
CO2 can help your plants grow faster. If you have increased you lighting they will need CO2 and extra nutrients. I find with 2wpg on my 40gal and DIY CO2 my plants are growing well with out extra nutrients. Also my nitrates are always at zero with a well stocked tank of tetras, so i'm sure they would benifite from extra nutrients too but i'm happy with they way things are now, and like i mentioned with out CO2 for 6 weeks growth def. slowed down.
Well i hope that answered all your questions and also there is a lot of stuff about this online so don't br afraid do to some googling