Mid-tank swimmers?

I think I already know the answer to this, which will be, "Don't add to your stocking," but I thought I would ask.
I have a 145 litre tank, a little cory army, 1 bristlenose pleco, and a pair of electric blue acaras. All get on, excuse the pun, swimmingly.
But all fish are all always at the bottom of the tank, snuffling, sleeping, cruising... so I have a lovely tank and beautiful 'show' fish that rarely move up into the mid section of the tank. Because my lovely corys mean that I can't use soil, the plants don't do terribly well and are currently only occupying the bottom third. Photo attached.
So I have a happy, healthy tank (water parameters are all appropriate and my acaras try to breed) which appears empty!
Any advice?

I’d put a black background on it to start with… @Colin_T says the fish feel safer, I didn’t know that but I like the way it changes the way the tank looks.
As for fish…not a clue

Don't think it would be impossible to add something new but how many cories do you have? A school of something like Lemon Tetras or Flame Tetras would be a nice impact in the middle and they get to a size not to be threatened by the Acaras.

Wills

Agree, the size of tankmates must be considered, as this cichlid (Andinoacara pulcher) attains 6 cm (15 inches) and although considered relatively peaceful it is still liable to eat small fish. Finding upper level fish is not always easy on its own; many of the peaceful characins (tetras here) prefer shaded waters and tend to remain in the lower half to third of the water level. Adding some floating plants would likely help, even with Lemon Tetras or Flame Tetras; otherwise these might remain lower down as well. Most characins have what some have termed a light phobia, and they avoid (in their habitats) any bright light, remaining in the shade, under plants (floating or overhanging terrestrial vegetation), etc. The cichlids might venture up a bit more as well.

As for the plant problems, this is not due to the substrate. Aquatic plants will grow in any substrate that does not have grains that are large, say pea gravel and larger. The light might be a factor, but more likely the nutrient level. Are you using any fertilizers, and if yes, which? A substrate tab would significantly improve the Echinodorus plant at the back, and the red-leaf plant on the left which looks like a species of lily (Nuphar or Nymphaea species?). Flourish Tabs are the best I have found. A comprehensive liquid might help all of the plants, and especially if you get some floating species which really will make quite a difference to the fish. As will a dark (plain black works very well) background, as mentioned already.