i think we both know Wilder, no matter how experienced you are, sometimes those type of fish, especially the livebearers can be perhaps some of the hardest fish to keep and keep well. I think my lowest points in terms of the actual fish, has been with livebearers. How about considering a complete change one day? Or moving the cory in with the rest, keep the 12 Gallon as an emergency tank and treat yourself to a new tank with a specific set up, like shell-dwellers, you sound like a Shell-Dweller person to me!!!
i am not saying sand essential to Cory, just my preference, and wondered what substrate you had