Ok, glad it is only your nitrates that are high. For now, I would just keep doing large pwc. But, 30% is not really enough in this case. I suggest doing several at least 50% changes to get your numbers lower.
How long have you had the cories? Both cories and mollies are somewhat sensitive to nitrates. I have definitely mollies have issues at 80 nitrates before, and cories too especially if the gravel has not been thoroughly vacuumed. It might be that something came in on your new fish though, but I am not entirely sure what. Would you be able to give us a more detailed rundown of how long you have fish, when you added more, when deaths started happened, and circumstances around the deaths?