Last time I was using this, my PH leveled
out around 6.8. Should this be enough CO2 for a 75 gallon?

Depends on your KH. If you don't know your KH, what pH is your tank without
CO2 injection?

Also, should I wait until my PMDD levels have leveled off and my PH is
stable from the CO2 before increasing light duration?

IME, no. Once you increase your lighting, your plants will use a lot more PMDD
and CO2. So you'll have to find the balance all over again anyway.


Leigh

http://www.fortunecity.com/lavender/halloween/881/