Awesome tank and an innovative idea. I don't think it's as unstable as it looks. Any air that gets in will only displace that amount of water. So if a litre of air goes in a litre of water will come out but as long as a free flow of air doesn't get in then the water wont 'flow' out of the holes.
If you seal the top section to the same standard as you would a normal tank it would be easily capable of sustaining air tightness, probably the same chance as getting a leak in a normal tank. Although there will be high compressive forces on the structure from the weight of water forcing downward so the glass/acrylic probably has to be well braced and quite thick.
They must close the feeding ports and have a hatch at the top to top up the water, put in fish etc though that then seals nice and tight.
