Thanks for your comment Burko.
Indeed, the separation is not something that I am happily doing in any way - it's a real pity to cut this window in pieces.
However, the aquarium connects to a terrace on the floor above (that's how the light is shining through it) - so if the water level would not cover the whole glass, it would destroy the water feature effect on the floor above. That's why I really want the glass to be fullly covered and the water level going even a few inches higher.
Pavel