Be careful, it might be dangerous. Also assuming you started with right salinity level, by the time you top off the water the salinity will be too high. High salinity will decrease available oxygen in water which is not good. Also, are you testing the copper concentration? The concentration might vary and you might either drop below or go beyond the recommended levels. This can either cause the copper to be ineffective or might potentially reach toxic levels.
For the theory of why not to add copper to top off water. You add RODI (pure H2O fresh water) to compensate for evaporation. When water evaporates, it leaves behind all the salts and chemicals including copper, so when you top off you are basically returning the evaporated water, the salt and the copper are still in the system so you dont need to add extra except when you are doing water change, in that case you are removing the salt and the medication, so when you add the new water, you should add back the salt and the copper that you took out of the water.