How can I establish how much CO2 I should add to the fishtank?

I have a large tank (100 gallons) with lots of plants and about 120 fish from Tetra Family. My plants are not growing well, I'm pretty sure is due to lack of CO2. I have a DIY CO2 set-up with 2 bottles, but have all the time leaks. I want to go for a better set-up via CO2 a charged CO2 bottle. I do not want to take the risk that I kill the fish with to much CO2. The question is, how do I establish the correct dosage for the plants, while not harming the fish.