Hi,
There's not very much information there for anyone to make a proper diagnosis.
Does your tank have a lid which will help retain CO2, do you have any form of surface skimmer which will aid in off-gassing, do you run an airstone overnight which will also aid in off-gassing, have you checked that your drop checker fluid is behaving correctly, there could be a lot of reasons?
Have you tried doing a PH profile, using as a baseline, water from your tank that has off-gassed for 24 hours and is the same temperature, this PH value is your starting point for a 1 PH drop?
I believe there are quite a few on here who experience residual CO2 but a lime green drop checker the following day appears to be a bit extreme to me.
Cheers!