I would suggest that 18C (64F) is much too cold for any Corydoras. I will consult Ian directly if I can, but I can find no references for this low a temperature. C. aeneus range in every source I have is 21-27C/70-80F, optimum around 24C/75F. C. panda is 20-26C/70-79F, with wild-caught fish requiring temperatures at the lower end of this range.