rising waters

I just don't get it, my tank water rises by itself.

I do a 20% water change on my 66g and I stop filling before the water
line reaches a DIY glass shelf which holds up part of my glass cover. I
then do my usual maintenance and when I'm finished, it all looks fine.
About an hour later, the water level has gone up and touched a portion of
the glass cover so that I can see it from inside the tank (drops below
the water surface). Several hours later and more of this glass portion
is now underwater. Either my aquarium is shrinking or the water level is
going up. After several hours, the waterline stops rising and stays
high, slowly dropping from evaporation until the next water change.

I know I'm really opening myself up to some snide comments (and yes, I
already know that I'm somewhat crazy), but I have watched this happen
many times and cannot figure out what the mechanism is. The water is
from my well, so it's under higher pressure and has more dissolved gases
in it (which is normal for well water). I've been using it from my tap,
so it's somewhat softened and at the right temperature (if that makes any
difference).

So water expands its volume when it outgases ? If anything, I would
expect the result of outgasing to be a net decrease in volume, but I'm
chemistry-challenged, and all I remember from physics is that liquids
can't be compressed (which apparently isn't correct).
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www.NetMax.tk