Are inverted aquariums in ponds harmful to fish? Is the air in the veins doing anything unusual? Can egg development be affected under these conditions?

I can't see why it would be harmful. What air in the veins? I don't understand what you are talking about there. Where in MN are you located?

Njn,

I have seen some inverted aquariums on the internet that are visually stunning! I suspect they would be much easier to maintain in a small "garden pond" right beside your house, compared to a larger rural pond.

I do not believe there would be any harmful effects on your fish if you added such a habitat to your pond. When people talk about using a vacuum to fill the aquarium with water, that is just part of the initial fill up process. You just stick a long wand up to the top of the tank and suck out all of the air. The earth's atmospheric pressure then actually holds the water in the aquarium above the water level of your pond. (If your aquarium has even the tiniest leak, then air will slowly get sucked into the tank and the water level will go back down to the level of your pond.)

However, I do not think the water in the tank would be particularly attractive to your fish. It will have poor circulation, and will therefore eventually have a lower content of dissolved oxygen if the fish do reside in it for extend periods. You could change that by adding some underwater plants! These plants would produce oxygen during the day when the sun was shining. You might be able to add that feature and make a stunning display in your own pond!

Good luck on your project.

I think I'll invent the Inverse Terrarium.

There must be a quite big negative pressure - something like decompression sickness?

Sometimes I think we're all living in one!

It seems to me that bubbles from plants, and from the pond bottom, would eventually displace all the water that is above the natural pond level, and maybe even more, making a negative level.