So sorry to hear of your losses, too, 1077! Honestly, it makes me feel a bit better than I am not the only one experiencing this. I totally understand about being picky with water quality; I really don't think that's the problem, but it is so frustrating not knowing what the problem is. I'll keep researching, too; my various batches of cories come from different places as well, so I don't think it's an issue of getting them all from an infected tank. I'll go ahead and get a new nitrate test kit just to be safe. Thanks for the response and the sympathy! Congrats on the baby angels!