With a pH of 7.8 and KH of 18, you should have a CO2 concentration of more or less around 8ppm. The usual stable CO2 concentration I see in most non-injected tanks is about 2ppm.
It may be a residue of what you've tried in the past to lower pH. Partial water changes won't completely eliminate things from your water... only complete water changes. Over time as you do more water changes, you will remove more and more of the substances, until very very little is left (and thus we round off it, saying none is left). I am unfamiliar with your method of pH lowering, so I cannot give you any advice on that.
HTH
-Richer
It may be a residue of what you've tried in the past to lower pH. Partial water changes won't completely eliminate things from your water... only complete water changes. Over time as you do more water changes, you will remove more and more of the substances, until very very little is left (and thus we round off it, saying none is left). I am unfamiliar with your method of pH lowering, so I cannot give you any advice on that.
HTH
-Richer
