Have you changed the lighting recently? I had to troubleshoot weird corner behavior with my chilis and the answer ended up being the light--I got one that ramps gradually up in the morning instead of just turning on all at once and that made a really big difference.
If you didn't change the light, my bet's on the filter. They're so teeny-weeny that the currents affect them more--they might just be in that corner because it's the easiest place to stay still and try to hunt nano-prey, which seems to be their preferred behavior in my tank. A slower current or more things in the open space of the tank to break up the current might help them spread out, if either of those things are workable with your setup.
If you didn't change the lighting and have no options with the filter, my last recommendation would be adding more floating plants so that there's less "open water" in the tank. Mine really like the floating water sprite plants I've added (one of which is from Aquarium Co-op! They ALL would be but I'm lucky and my LFS has the broad-leaf variety, haha), and sometimes if you get them comfy with lots of plants you can slowly remove them without freaking out the chilis, if you're not into the HEAVILY planted aesthetic that they prefer. Good luck!
