i have a 5 month 29 g tank, with co2 and ventri reactor, over the last 2 weeks am having problem to keep the co2 high, am using a dupla co2 cheeker , the ph will droop but the co2 idicator turns light green at ph of 6.3, before that it usuly turn yellow at this ph ? am using a ph prop which i cilibrated and it seems acurate?
what could be going on?

You should have known 4 dKH distilled or DI water in that Dupla CO2 checker, with a drop or two of pH reagent mixed in. You should not have tank water in the CO2 checker. Once you do that, the CO2 checker color will be green at about a pH of 6.6, which means about 30 mg/l of CO2 in the water. The tank water may have a pH of just about any value between 5.5 and 7.5 at that time, depending on the KH of the tank water, and what other substances are in the water, from tannins to phosphates.