If you're dead set on DIY CO2, then you need to get a CO2 drop checker. It's a very cheap and efficient way of testing CO2 levels in your tank (you shouldn't dose anything you can't test for). That way you can see when it's time to change your bottles, if you need to add more bottles, or if you need to start aerating at night.
Ok OK NEED help here the lights that is have are T8 15watt on the box says = to 75watts, so when we talk 1 watt per gallon are we looking at the 15 or the 75 i just dont understand???