Aeration doesn't remove chloramine so that may be why you saw ammonia.
That pH swing is a bit much for overnight CO2. You could add sodium
bicarbonate to give a bit of KH to buffer the pH but my tanks run at pH 6.4
with less hardness than yours ( 3 k, ~10 GH) and I just keep a piece of
tufa rock in the filter box to hold up the kH a couple of degrees.
A bit more surface agitation to enable CO2 to escape may do the trick ;o)

Sue