I see what you mean.
The school of corydoras catfish is definitely a great sight to watch. The Cardinal Tetra are not feeling well to come out of the corner. It is likely they do not feel safe enough. There is also the possibility that they like the warmer corner. The first thing I would do is to move the heater away from the corner. It should be placed toward the middle of the back. Relocate the tree house decoration to the corner to take over that part of the free space might force them to come out. You might want to try it too.
By the way, what is the black tube at the bottom below the heater? Is it the intake of the filter?
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20 gal tank - good mix of community fish ?
You might want to try it too. 
