Ideally the CO2 concentration should stay consistent throughout the photoperiod even with plants in the tank. It takes a drop checker (properly filled with 4 dKH water) a couple of hours to register. If your circulation is good it only takes a fraction of that time for the CO2 to build up to the proper level so a drop checker is always "behind schedule".
Do you have any surface agitation? If there is no surface agitation that would be a reason your drop checker is staying yellow after the gas is turned off. Without some surface agitation or an airstone running the CO2 will be very slow to leave the water.
Some surface agitation is good, you need O2 in the water too and the more O2 the fish have available to them 24/7 the higher the level of CO2 they can tolerate. There is no reason you can't have adequate levels of CO2 and O2 in a tank simultaneously.
Do you have any surface agitation? If there is no surface agitation that would be a reason your drop checker is staying yellow after the gas is turned off. Without some surface agitation or an airstone running the CO2 will be very slow to leave the water.
Some surface agitation is good, you need O2 in the water too and the more O2 the fish have available to them 24/7 the higher the level of CO2 they can tolerate. There is no reason you can't have adequate levels of CO2 and O2 in a tank simultaneously.
