when a plant isnt doing well, 90% of the time, its a CO2 issue (try adding more). do you know approximately how much CO2 you have in the tank (drop checker? kH - pH comparison?)
with DIY CO2, the rate of CO2 release is somewhat unstable, because the population of yeast is constantly growing (til it crashes), and because their metabolic rate is dependent on the temperature. instability is bad for plants, some are more sensitive then others.
i would do 3-4 half-liter bottles, and change one per 5 days (rotating). actually, i would (and did
) use a pressurized CO2 system, but i understand that it can pose a financial issue.