I would just top it off daily
And stay with your weekly water change ... Evaporation is something that happens.. But if the calcium content of your water is that high.. then it is best to keep the tank filled everyday.. Then you will not have to worry about the calcium lower than the taop 
Plus it is a smaller tank ... So filling it daily will not really raise your water bill.. I wouldn't think anyway.
Oh and a razor blade should help
Plus it is a smaller tank ... So filling it daily will not really raise your water bill.. I wouldn't think anyway.
Oh and a razor blade should help