Good luck! .....just kidding...but that will be quite a task. If you can remove anything like hard scape it will help. I would get someone to help...one of you on each end of the tank. The idea being to run them to whichever end you want to catch them on. I always find that helpful.
Specimen container for fish. Set at the bottom of the tank, keep one hand on it, herd the cories that way, and then lift it up and out when you have captives.