I would recomend getting the 20lb tank for the simple fact that they are much easier to transport and handle. I just filled mine yesterday for $15. One tank lasts me about 45 days. I have my flow meter set at 1.0 and it is on for 15 mins then off for 15 mins. The CO2 is off during the dark period. I have the formula that came with the CO2 system but I'm not sure where it is. I'm sure someone here can post it b4 I could find it.
I would also recomend that you check your flow periodically by placing the tubing in a glass of water and check the flow of bubbles. I say this because the selonoid on mine stuck shut once and my plants weren't getting anything. Also the little matal ball inside the flow meter will sometimes stick at the bottom and show zero flow although C02 is indeed flowing. A little tap on the side of the meter and the ball jumps up to where it should be.