I know your tank is fairly typical of many, but I think from the perspective of small fish, it's really quite bare without much shelter to make them feel comfortable. If you are worried about going in for live plants, I would suggest that you start by attaching Java fern to some of the objects in the tank, it may be all that you need.
I also wonder about the number and type of fish and if more of a similar type of fish would make the X rays more confident. I know you have tried this with the Pygmy cories, but they are a very different type of fish. I bought several used tanks a while ago and some of them came with a few small fish, mainly tetras. At first I put each type into a separate tank, planning to add more of each later. Under these circumstances, some of them were very shy and would hardly ever be visible, in fact in the case of Black neons and Black phantoms I really thought they must have died several times as they were impossible to find. After a while, I decided that more of these were not in my plans and so I put them all in one tank. Within a day or so, the behaviour of all of them changed, they all became much bolder and shoal together, never hiding away. The Black neons went from being total recluses to the boldest fish in the tank. This small 60 litre tank contains mainly tetras, 12 Neons, 2 Black Neons, 4 Cardinals, 2 Black Phantoms and 2 Rummy Nose plus one male guppy. They all shoal and mix together happily, even the guppy. Incidentally, the tank is 120% overstocked according to the calculators but they have been in there for over a year with no problems at all with only a standard filter and minimal water changes, though with lots of, live plants.
What has this to do with the question? I reckon that if you had more similar fish (i.e. more tetras) in the tank, they would all be more confident. These fish all live in large shoals in nature and the more there are, even if not all the same species, the happier they seem to be.
It's possible your fish had spent all their live in bright light e.g. at the fish shop and then you brought them into another brightly lit environment to which they were already accustomed. When the lights were off they had their first taste of the conditions they really like and want more of the same! That may be nonsense, but I can't think of another explanation.